Superpowers can be something you're born with or something you're accidentally afflicted with. Rarely do they come from something you can eat, but this is exactly how the characters of One Piece are able to empower themselves with weird and wonderful abilities. In the long-running manga/anime series, Devil Fruits are the strange source of many a hero and villain's power, including the franchise's straw-hatted protagonist, Monkey D. Luffy.
There are rules (and exceptions) to what happens when someone ingests a Devil Fruit. One big drawback is that the user can no longer swim and faces a real risk of drowning -- which is a severe problem for a world populated by pirates and Marines -- though this weakness has slowly changed throughout the series. Users can only ingest one Devil Fruit at a time (with one prominent exception). When the user passes away, the Fruit they had ingested reappears in the world for another to possibly eat, which is how Sabo was able to acquire his adopted brother, Portgas D. Ace's Flame-Flame fruit. There are also "artificial" Devil Fruit called SMILE.
Before we detail the rules and exceptions in further detail, let's take a look at what Devil Fruits are and what variations they come in, starting with their three types: Paramecia, Zoan and Logia.
PARAMECIA
The Paramecia Devil fruit is "the superhuman type" and it is the most common. Powers acquired with the Paramecia can be further separated into three categories: physical, bodily changes environment manipulation or substance production. The most famous Paramecia Fruit user is none other than Luffy himself, who ate the Gum-Gum fruit. It changed Luffy's physical body, which he can now stretch like rubber. Since his body always has this rubber consistency, his power is always "active."
Another type of Paramecia allows external modification of the environment around the user. For example, these Fruit users simply need to touch what they wish to change or affect. The Clear-Clear fruit, previously ingested by Absalom then transferred to Shiryu upon Absalom's death, grants its users the power to turn themselves invisible and or anything they happen to touch. Paramecia types can also create and control different substances, depending on the Fruit eaten.
Other notable Paramecia users besides Luffy are Nico Robin, Brook, Trafalgar D. Water Law and Donquixote Doflamingo.
LOGIA
Logia Fruit users have an elemental transformation. While some Paramecia types can create and control different substances, similar to an elemental transformation, Paramecia Fruits users are unable to become their element.
Notable Logia users are Portgas D. Ace, Crocodile, Caesar Clown and Smoker.
ZOAN
Zoan Fruit users have an animal transformation, with variations of single species or animal hybrids. This transformation is also possible at will. The Zoan Fruit user types have three forms: human form (or their natural form), human-beast form (a hybrid form) and a beast form, or their fully transformed state. Some users are able to activate new transformations, like Chopper's different forms. There are also different categories within these users separated by the animal species of their transformation, such as the Dog, Ox and Snake series.
Notable Zoan users are Rob Lucci, Tony Tony Chopper and Kaku.
DEVIL FRUIT RULE EXCEPTIONS
Artificial Zoan Devil Fruits, or SMILE, were created by SAD, which is a substance created by Caesar Clown. Those who ingest the SMILE Devil Fruits still have the weaknesses of other Devil Fruit users and there is only a 10 percent chance of success in obtaining an ability from them.
Within the Paramecia type, there is an exception with Katakuri Charlotte, who is one of the Three Sweet Commanders of the Big Mom Pirates. With his Mochi-Mochi fruit, he can create mochi-like substances and become mochi-like. Within the Logia type, the Dark-Dark fruit, eaten by infamous Marshall D. Teach, otherwise known as Blackbeard, allows him to control darkness and gravity; however, Blackbeard's body does not reform after attacks or become intangible.
Blackbeard is also one of the biggest exceptions of them all: he has two Devil Fruit powers. One is the Tremor-Tremor, or Quake-Quake fruit, and the other is the Dark-Dark fruit. The latter he ingested first while the former he was able to eat following Edward Newgate, or Whitebeard's, death. There are many theories on how Blackbeard is able to utilize two Devil Fruit powers, the main clue being his knowledge from studying the Devil Fruit encyclopedia. However, as series creator Eiichiro Oda has not yet revealed this in the series, we're still none the wiser.