Newbie / git / gitosis question

I've been working away at Gitosis and it's mostly fair enough but
there's one bit that's unclear to me...

git push origin master:refs/heads/master

Would somebody kindly explain (or point to docs) what
refs/heads/master means? How is this different from just 'git push
origin master' or even 'git push origin master:master'?

Any insights much appreciated.