Captain Marvel is the alter ego of a kid named Billy Batson. Even though Billy Batson is a little kid, all he has to is say the word, “Shazam,” and then he is magically transformed into the adult Captain Marvel, who is pretty much like Superman. He wears a red suit and has a cape. Billy uses this superpower so he can get into R-rated movies and buy beer and fireworks.
Captain Marvel is actually older than Marvel Comics; it was published by Fawcett Comics. In the 1950s, DC Comics sued Fawcett because the character was such a big rip-off of Superman. Later on, DC bought the rights to publish the story again.