The man page for git-config lists these options for push.default:
nothing - do not push anything.
matching - push all matching branches. All branches having the same name in both ends are considered to be matching. This is the default.
upstream - push the current branch to its upstream branch.
tracking - deprecated synonym for upstream.
current - push the current branch to a branch of the same name.
In most cases I would assume that pushing to a branch's upstream branch would be the same as pushing to a branch of the same name, since the upstream branch would normally have the same name, and since the branch of the same name ("current") would normally (or always, by definition?) be upstream. So what's the difference?
UPDATE: The man page for git-config has been updated (as one would expect), so the distinctions made there may be a lot clearer now.