SPOILER WARNING: This article contains spoilers for Tuesday night's episode of "Agents of S.H.I.E.L.D."
Johnny Blaze was a stunt motorcyclist who performed in a stunt show ran by the aptly named Crash Simpson, whose family raised Johnny since he was a child. During this time, Johnny fell in love with Crash's daughter, Roxanne. When Johnny discovered that Crash was dying from cancer, he sold his soul to the devil so that Crash would survive the cancer. The devil came through...only for Crash to die in a crash soon after. However, Roxanne's love for Johnny was so pure that the devil could not take his soul, as it was attached to the purity of Roxanne's love. So the devil did the next best thing, it bonded Johnny to the demon Zarathos, and the ensuing result was Johnny turning into the blazing skull flaming motorcycle-riding Ghost Rider.
Johnny split with Roxanne to protect her, as he was forced to turn into the Ghost Rider and take vengeance whenever he was near bad people. This kept up for many years, as Johnny traveled the country to try to find as much peace as he could. Eventually, he was able to split from Zarathos. However, over the years he has returned to being the Ghost Rider over and over again, to the point where he currently is just at peace with the fact that he shares his body with a demon.
Dan Ketch
At first it appeared as though Danny Ketch became Ghost Rider simply by finding a motorcycle with a symbol on it that Danny touched and activated, becoming the new Ghost Rider. Unlike Johnny Blaze, this new Ghost Rider was a more benevolent-seeming demon, although its methods of vengeance could often be ruthless, as well, like its Penance Stare, the ability to look a villain in their eyes and force them to feel whatever pain that they've put out there into the world. Eventually, Danny and the now-human Johnny Blaze teamed up and formed a partnership together. Over time, they learned that they were secretly brothers (their mother had taken her two youngest children and started a new life for herself and Johnny was told that she had died).
Danny's Ghost Rider, as it turned out, was Noble Kale, whose soul was stuck between Heaven and Hell, and the alternative that they both came up with was to enlist him as a spirit of vengeance. Kale's ancestors would be destined to merge with Kale and become Ghost Riders for generations. Dan Ketch was the last of his line to become Ghost Rider, as the two ultimately came to an accord of sorts as Ghost Rider. Ketch then lost his Ghost Rider abilities but they later returned to him. Ketch is mostly laying low these days.
Alejandra Jones
Alejandra Jones had the worst childhood of all of the Ghost Riders, and these people have had some rough childhoods between them. In her case, though, her own father sold her into slavery. She ended up being purchased by Adam, who turned out to be the Biblical Adam, but was obsessed with ridding the world of sin, no matter what the consequence. Alejandra was one of a group of other slave children raised to train themselves to possibly become the new Spirit of Vengeance. Eventually, Adam managed to convince Johnny Blaze to sign his curse over to him and Adam transferred it to Alejandra.
Alejandra tried to do her best as the new Ghost Rider, but ultimately her inexperience proved to be a major problem for the rest of the world, as she was frequently easily manipulated. Blaze offered to train her, but in the end, betrayed her as part of a plan where he would regain his curse from her. In the end, she survived due to having a small part of Blaze's power, but she then vowed that she would get her vengeance on Blaze one day.
Robbie Reyes
The current, all-new Ghost Rider, came about in a similar fashion to Dan Ketch, in that he had a relative who was not a demon but whose spirit was bonded to Robbie. In Ketch's case, that spirit was Noble Kale. In Robbine's case, it was his serial killer uncle, Eli Marrow. Robbie was a mechanic who would often borrow cars that he was working on so that he could race them in illegal street races, He had almost enough money to move himself and his younger brother (who had been injured in a racing accident) into a new life for them both when he discovered that some bad guys were after something he had in the Dodge Charger that he had borrowed. They shot him to death, but before he could die, the car actually spoke to him, offering him a chance for vengeance. Robbie agreed and bonded with the spirit (who, as it turns out, was his uncle, Eli).
As the Ghost Rider, the Dodge Charger also became a Hellcharger. Ultimately, Robbie and Eli began to solidify their relationship. Robbie would kill, on occasion, for Eli, but only people who deserved it. Robbie was given training by Johnny Blaze, who learned that there was a new Ghost Rider in town. When they split, he promised Johnny that he would turn to him if he ever needed help.
Vengeance
If you wanted an idea of how grim and gritty the 1990s could get a time, just note that Marvel decided at one point that Ghost Rider, a flaming skull guy in a burning motorcycle who could drive people insane with a penance state, wasn't quite edgy enough. So they introduced Vengeance. Michael Badlino was a cop who was a supporting cast member in the Dan Ketch "Ghost Rider" series and when Johnny Blaze accidentally drove Badilino's father insane with a blast of hellfire (Blaze had taken to carrying around a shotgun that shot hellfire). Badilino then struck up a deal with Mephisto that he would be turned into a spirit of vengeance to get his revenge on Blaze.
Ultimately, Badilino realized that Blaze had been manipulated into doing what he did and Vengeance became a good guy, even filling in for Ghost Rider when he was missing for a time. Vengeance, though, was slightly edgier than Ghost Rider and was not averse to killing people. Eventually, Vengeance devolved to the point where he was just murdering criminals willy nilly. He ended up sacrificing himself and ended up in hell.
Caleb
A former slave who survived the horrors of slavery and the Civil War only to be murdered along with his family by a band of racist outlaws, Caleb (last name unknown) returned for vengeance as the Ghost Rider. He teamed up with Travis Parham, a former Confederate soldier who had been near death when Caleb had rescued him and nursed him back to health. When Caleb was later killed, Parham swore vengeance on his friend's killers and hunted them down, not knowing that the mysterious flaming head guy on a horse that was also hunting them down was, in fact, Caleb. Caleb's only notable appearance was in the mini-series "Ghost Rider: Trail of Tears #1" by Garth Ennis and Clayton Crain.
Naomi Kale
Johnny Blaze and Dan Ketch's mother, Naomi Kale, did her best to try to avoid ever turning into the Ghost Rider during her life, as she was the latest person in her family to carry on the arrangement with her ancestor, Noble Kale. She used spells to hide herself from Mephisto, who could help force her into taking on her role as Ghost Rider. She also split from her family to protect them. Her plan was basically to make it so that she would just stay away from them and keep the curse to herself until she died. She turned to drugs to help her through her decade away from her children. Eventually, though, she sadly became deathly ill. She visited her children briefly before she was then challenged by Mephisto, who threatened to kill everyone her family loved (her actual children were protected from Mephisto). She agreed to become the Ghost Rider one last time, and thus her power was placed into a medallion which was left with her motorcycle, which would be found years later by her son, Danny.
Max Parrish
Max Parrish was an archaeologist who only appeared in, of all places, a mini-series starring Susan and Johnny Storm from their days before they became members of the Fantastic Four, aptly titled "Before the Fantastic Four: The Storm". The series was about a Medallion of Zarathos that killed Parrish's brother-in-law while he was studying it and when it ended up in Johnny's possession, sent a whole pile of black magic bad guys after the Storms. Parrish helped them out and when Johnny was given the chance to become the host of Zarathos, Parrish stepped in instead. Zarathos was too much for Parrish to handle, however, so Johnny and Susan luckily were able to force Zarathos to give Parrish up when Johnny tore the medallion from the Ghost Rider's motorcycle and threw it away.
Red Hulk
During the crossover "Circle of Four", Ghost Rider (Alejandra), Venom (Flash Thompson in his Agent Venom time), X-23 and Red Hulk all teamed up to fight Blackheart, who was trying to literally bring hell to Earth. They were more or less screwed until they came up with a gambit that not even Blackheart could have expected - Thompson let his symbiote bond with the Red Hulk and Alejandra then temporarily passed her curse to Red Hulk, so he was a Ghost Rider Venom Hulk. It didn't make a whole ton of sense, but when it was something as amazing as Ghost Rider Venom Hulk, it did not need to make a ton of sense.
Wileaydus Autolycus
Wileaydus Autolycus grew up in the distant future and was raise to hate the Universal Power of Truth (which was fair enough, as the Universal Church of Truth was pretty evil). Through mysterious means, he gained the abilities of the Spirit of Vengeance, which is what he called himself. Since he is an alien, it is unknown what connection, if any, he holds to Earth's Ghost Rider. He ended up becoming an ally of the Guardians of the Galaxy and even joined their spin-off group, the Galactic Guardians. Remember, this was the early 1990s, when every comic book had to somehow work in Ghost Rider, Wolverine or the Punisher into them, even comics set in the 31st Century (the series did, in fact, also manage to sort of work in the Punisher and Wolverine into the series, as well).
Kenshiro "Zero" Cochrane
There's no real connection between "Ghost Rider 2099" and the normal Ghost Rider. Zero Cochrane was a hacker who was caught and shot for stealing from an evil corporation. Before he died, he managed to download his consciousness into cyberspace where he was enlisted by a mysterious group known as the Ghostworks to be their agent in the physical world. He downloaded his mind into a robot body and fought crime as the Ghost Rider (and also sought out vengeance on the corporation that had him killed - a corporation where his own father worked!).
A Whole Pile of Ghost Riders
During Jason Aaron's run on "Ghost Rider", he revealed that there were actually a whole mess of other Ghost Riders and that the evil angel Zadkiel was hunting them down (using a manipulated Danny Ketch as his agent) and taking their power so that Zadkiel could have enough power to storm the gates of Heaven. Ultimately, once Ketch was freed from Zadkiel's control by his brother, Johnny Blaze, the two Ghost Riders took on Zadkiel with a whole army of Ghost Riders. Since Aaron's run, the whole "There are Ghost Riders all over the world" aspect of the Ghost Rider concept has been downplayed significantly, although the existence of Robbie Reyes does suggest that Marvel is open to the concept.
There will almost certainly be more Ghost Riders in the future (we're not even counting the alternate reality Ghost Riders that appeared in the "Secret Wars" mini-series, "Ghost Racers") and we can't wait to see them all!