I'm glad you asked. It is a very large event involving DC comics, home of superheros like Superman, Batman, Wonder Woman, Green Lantern etc. In the comics, all of the above mentioned figures interact with each other on a regular basis, and appearin each other's titles. Superman might be in Batman Comic, Batman in Green Lantern, and so on. They, plus many other hero's, form one large story known generallyas The DC Universe and it has been going on for years and years. Imagine it this way: It's would be like if all the characters on every sitcom on televisionall knew each other and interacted with each others shows to varying degrees. Jerry Seinfeld goes over to Drew Carey's house for dinner, and ever now and then they team up with Chandler from Friends and go to a strip club together. It's sort of like that. Episodic.
    Infinite Crisis is a mini-series that involves all of these people, meaning Superheroes not TV Sitcom charactersfrom the 90's, and is the apex of a very long story. Many people in the books will die, major plot lines will be resolved, and the story will never be the same ever again.
    So it's a big deal.
    If anyone wants to know anything else about the subject matter, I suggest you visit the following web sites. They should give you a pretty good Idea of what is going on.

    Wikipedia: Infinite Crisis
The following two links explain about the thing this is a sequl to, Crisis On Infinate Earths:
    http://www.moviepoopshoot.com/comics101/8.html
    http://www.moviepoopshoot.com/comics101/9.html