One Flag CTF has only one flag, and it's in the middle of the map. You must grab the flag and take it into your opponent's base to their flag area to score a point. Overload contains the same two flag areas in each others' bases, but instead of a flag, there's a large steel skull in its place. You must destroy the skull, which is no small feat as it takes a lot of damage. Every time you do this you score one point.
Harvester is the most original mode of play.
Every time someone on your team kills an enemy, a little mana skull appears in the center of the map. Your team must then collect these skulls and take them into your opponent's base to score points. You get one point for every skull you successfully bring in. I had the least fun in Capture the Flag. I don't think it was necessarily the game's fault, but CTF has been done.