Converting commits to patch files? HEAD vs HEAD^

Jeff Garzik's guide doesn't appear to explain how to get patches back
out of the system.  

I've successfully commited a set of changes.

 # git diff HEAD^ HEAD

This command will produce a diff of the changes I've made.  What is
the HEAD^?  Does it refer to the commit before the last one made?

If I've made several commits, I'd like to be able to gather several
together and produce a patch file.  Better still, I'd like to be able
to pick a set of discontiguous commits an bundle them into a single
patch.  Ought I be using tags?

Finally, given that the upstream repository is git, what is the way to
push commits upstream?