Achievements

I just noticed, that when you complete an achievement sometimes that achievement pops up at the top of your screen telling you to go and collect your reward. Also I noticed that it often said , "achievement complete" and then" 5000 experience gained" or something like that. What is that experience for? Or go towards?  It's obviously not in game player experience points. 
Any help is appreciated. It's driving me nuts that I can't figure out what that is for .