Things you (probably need to know) about the various characters in Avengers if you have not seen the other movies...
1) Thor & Loki - Thor is a God from the realm known as Asgard. He is one of two sons of Oden, the ruler of Asgard. Oden's other son is the god/magician Loki, who Oden actually stole from his enemies, the Front Giants, when Loki was just a baby. Loki tried to overthrow his father but was defeated at the end of the Thor movie. In defeating him, Thor destroyed the Bifrost, a magical bridge that allowed the gods of Asgard to travel among the various dimensions. As a result, it is sorta unclear how Thor will travel back to Earth. As the Bifrost was destroyed, Loki fell into it, making his fate unclear. At the very end of the Thor movie, there is a scientist on Earth who is asked to examine a mystical looking box. As the scientist looks into the mirror, we see Loki's reflection, implying that Loki has taken control of the scientist in some way.
2) Captain America - This movie takes place during WWII. Captain America, Steve Rogers, was a weakling who could not even volunteer for the army, but he is really tough and has a strong will. He agrees to take part in an experiment using a special serum to make you stronger and he turns into a super solider. The serum is destroyed in a sabotage incident moments after it is given to Steve Rogers. At the end of the movie, Steve saves New York from being bombed by piloting a bomb-filled plane into a frigid arctic snowbank. He freezes there and is thawed out 70 years later by Nick Fury and S.H.I.E.L.D. to become part of The Avengers team.
3) Iron Man - you probably saw this movie but Iron Man is the reckless Tony Stark, a billionaire playboy who is a genius at weapons design. He is badly hurt during a kidnapping incident and can only survive by "powering" his heart through a special nuclear reactor that he builds into his chest. It is this reactor that powers his Iron Man suit. He is not a guy who likes people in "authority" and has been fairly resistant to joining the Avengers or giving the government any of his Iron Man technology.
4) Hulk - Though there have been 2 Hulk films in recent years, it is not clear if the story from either one of them will have anything to do with the current character. About all you need to know about The Hulk is that David Banner is a mild-mannered scientist who you do not want to make angry.
5) Nick Fury - Played by Samuel L Jackson, Fury is the leader of the secret government agency S.H.I.E.L.D. His back story has not been explored much in the movies.
6) Black Widow - Played by Scarlett Johansson, she is a leather-clad assassin/spy. A martial arts expert, she had a decent-sized role in Iron Man 2, mostly just to kick bad guy butt. Her back story has not been explored much either.
7) Hawkeye - Played by Jeremy Renner, Hawkeye is a master marksman with a boy and arrow. He carries all kinds of special arrows in his quiver. He made a brief appearance in the Thor movie, but has not been discussed much in previous movies.
As an aside, Thor's love interest from his film, played by Natalie Portman, is not believed to be in the Avengers movie. However, Iron Man's beautiful assistant Pepper Potts, played by Gwyneth Paltrow, is in the new movie.
-Jason "I am seeing Avengers next Thursday and will post a review several days before it hits theaters" Evans