For those of you that live under a rock, The Avengers is a team of superheroes consisting of Iron Man, The Hulk, Captain America, Black Widow, Hawkeye, and Thor. Most of these characters have had stand-alone movies on their background. Poor Black Widow and Hawkeye are the exceptions. (Though Black Widow is quite important in Iron Man 2 and Hawkeye makes a cameo in Thor.) If you are going to see The Avengers, you need to see the previous movies to understand everything. The main villain comes from Thor while the major 'artifact' comes from Captain America.
First, let me say that this movie is awesome. I almost didn't want it to end. One of the reasons that I liked the movie is that it did require you to watch the previous background movies. That means that there wasn't time wasted on questions like "Who is this Thor guy and why does he carry around a hammer?" The movie was able to get right into gathering the heroes together and establishing why they couldn't fight this battle separately. Since we already know that they are powerful on their own, it is very important to show why they need to work as a team.
That said, there are a few little nitpicks I had. I always have a problem when they have a character on the screen for long periods of time without naming them. Nick Fury had this female 'sidekick' in S.H.I.E.L.D. that they don't name. My husband claims that they said her name once in the very beginning of the movie but I don't remember it. She goes through the entire movie as 'that chick that is the second in command or something.' Things like that don't detract from my enjoyment of movie. I would just like someone on screen to say her name more than once so I can remember who she is.
So, in the end, if you enjoy superhero action movies, you'll love this. There is a reason that the movie has been a blockbuster hit. I'm just sad that it'll take a long time before we see another Avengers movie.