Use the delete operation when you want to remove a file or directory from the repository.
If you try to delete a file which has been modified in your working copy, your VCS might complain.
Typically, the delete operation will immediately delete the working copy of the file, but the actual deletion of the file in the repository is simply added to the pending changeset.
Recall that in the repository the file is not really deleted. When you commit a changeset containing a delete, you are simply creating a new version of the tree which does not contain the deleted file. The previous version of the tree is still in the repository, and that version still contains the file.