Over at softwarecave there's a nice post on using git stash. In git, the stash is basically a stack on which you can push your current changes while you so something else. Maybe you want to change branches without committing your changes or perhaps you need to get rid of your current changes and work on something else in the current branch.
The post covers how to manipulate the stash. It turns out that it's remarkably flexible. If you're an Emacs/Magit user, the stash is usable from the magit-status buffer. If you're a git user, and especially if you're a Magit user, you should check this post out. It will make your life and work flow easier.