If you've made some changes, need to stop what you're doing, and not commit, you probably want to use git's stash feature, which saves your changes and restores you to a clean copy of HEAD.
Here's how to do it:
git stash save
...saves your local changes.
git stash apply stash@{0}
...restores your local changes.
git stash drop stash@{0}
...discards your stashed changes.