Happen is all ODI's. There's a bowling and batting powerplay, they are basically the same thing though.
Basically the first 10 overs are a the first powerplay. You're only allowed 2 fielders allowed outside the 30 yard, I think that's the distance, marker allowing for more boundaries.
After the initial 10 overs each captain gets to choose a period of the game where the same rules apply as before but for only 5 overs. Basically each team has 5 overs of powerplay to use.
The batting team will wait to take it when it suits them best and the runs are flowing whereas the bowling team will want to get them out the way.
Hopefully that makes sense.