Well, for call of duty 4 and call of duty WAW, Pretty much everything you have earned is gone. This means that all the guns and attachments are gone, all the challenges are gone. It is like starting all over at level 1
In modern warfare 2 you get new challenges and you can unlock titles and emblems I think, that you only get if you use prestige. It`s exactly the same like in modern warfare, but your titles and emblems doesn`t reset.