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 <commit 1> <commit 2>

If you want to remove untracked files (e.g., new files, generated files):

git clean -f

Or untracked directories (e.g., new or automatically generated directories):

git clean -fd