If you want to revert changes made to your working copy, do this:
git checkout .
If you want to revert changes made to the index (i.e., that you have added), do this. Warning this will reset all of your unpushed commits to master!:
git reset
If you want to revert a change that you have committed, do this:
git revert ...
If you want to remove untracked files (e.g., new files, generated files):
git clean -f
Or untracked directories:
git clean -d