What determines the EPL's spots in the europa league/uefa cup?

i have always wondered this in 07-08 only fifth place in the table made it to the europa league then in 08-09 and 09-10 it was 5th 6th and 7th place that made it so i thouh it was going to be like that for good no and now this season it went back to only fifth place. why is that?? is it like the world cup if your representation of your region does bad you lose a spot?? did England teams do bad in the last europa league ( i cant remember). if not then what is the reason??