Marvel Comics and Marvel Productions have been years setting up for this movie, beginning way back with the second Hulk movie, and continuing on through both Iron Man films as well as last summer's Thor and Captain America.
You do not have to have seen any of those movies to enjoy The Avengers.
If you have even the most basic knowledge of pop culture gleaned from TV (or even hearsay) you will seriously enjoy this movie. You don't need to know anything about Marvel Comics characters, really, because Joss Whedon, the director and co-writer, brilliantly explains who they are without unnecessary (or boring!) exposition. Briefly, Iron Man is a billionaire playboy inventor who created a suit of armor for himself so as to prevent a piece of shrapnel from getting to his heart. Thor is...Thor, the Norse god of thunder and brother to Loki, the Norse god of mischief. Captain America was the first -- and last -- recipient of an experimental Super Soldier formula back in WWII, who fought Nazis but was accidentally thrown into suspended animation during the War, only to be revived today, 70 years later. Hawkeye and Black Widow are master spies and assassins. And the Hulk is that same big green guy who pops up when Dr. Banner gets angry ("You wouldn't like me when I'm angry!") that we all probably remember from the old TV show.
Frankly, I didn't even need to tell you even that much. You don't need it.
What you need, is to go see this movie. It is most definitely a summer action popcorn movie, not an art film. But the action is absolutely breathtaking. The characterizations are wonderful, the acting quite good (especially Mark Ruffalo as the Hulk's alter ego, Bruce Banner) and the action sequences are without equal.
It is, and I do not say this lightly, the best superhero movie I have ever seen. Up until now that spot was reserved for the first Iron Man movie, which I still love. But Avengers blows it out of the water.
In fact, it is probably one of the best movies I have ever seen, period. I rank it up there with the first Star Wars (the really first one, Episode IV, not the execrable Episode I) and Forbidden Planet and Raiders of the Lost Ark.
It's that good.