The Working Tree in Git is a directory (and its files and subdirectories) on your file system that is associated with a repository.
It’s full of the files you edit, where you add new files, and from which you remove unneeded files. Any changes to the Working Tree are noted by the Index (see below), and show up as modified files.
When you open the files for a project that is being managed as a Git repository then you are access the Working Tree.
Tags and HEAD
Tags and HEAD are the same thing as branches: Pointers to a commit. The difference is that tags never move, even when you commit -- they're fixed pointers to a single commit object in the tree.
HEAD, on the other hand, moves a lot. When you commit, it moves to point to that new commit object. When you git checkout another branch, it moves to point to the same place that branch is pointing (...mostly). You can even git checkout individual commits or tags, and your HEAD will point to those commits.