In football, what does it mean when a player is placed on "Injured Reserve"?

I ask that question because I was reading an article in our local paper, and they mentioned that Ahman Green was placed on injured reserve with the Texans, and they said it meant that he would likely not play for the Texans anymore. They didn't really explain what it meant though; can anyone explain it in laymens terms? It didn't say that he was fired, just that he was placed on injured reserve. Is it the same thing as being fired? I'm disappointed because I don't feel like they even gave him a chance; he is really good, he just had injuries. When he wasn't injured he was awesome.