I didn't think there was any. I am sure that Yahoo took over Overture and then it became YSM. I could be wrong though. I only come to this assumption as I don't really hear many people talk about Overture anymore.
It's one and the same. Yahoo bought out Overture a while ago, renamed it Yahoo Search Marketing. Except some websites you can still see Overture... such as the bid overture tool.
I think overture is managing the yahoo part of pay per click... the keyword tool stats that overture has are coming from yahoo searches