The Fantastic Four are a group of superheroes from the comic book of the same name published by Marvel Comics.
Origin
The Fantastic Four are the result of a gamma radiation storm that enveloped their spaceship. After the storm, the founding members of the Fantastic Four developed powers.
Founders
As the name implies, the Fantastic Four has always had four core members. The four core members of the Fantastic Four are Reed Richards, Susan Storm, the Human Torch, and the Thing.
Past Members
While the Fantastic Four has always had four members, it has not always been composed of the four founding members. Past members include Crystal, Medusa, Power Man, Frankie Raye, She-Hulk, She-Thing, previously Ms. Marvel, (and the current writer is not able to name others.)
Arch Enemies
The main arch enemy of the Fantastic Four is Dr. Doom. The current author knows there are more, but can't think of them.