Working trees: They are nothing but the files that you are currently working on.
HEAD: HEAD is a pointer to the branch or commit that you last checked out, and which will be the parent of a new commit if you make it. For example, if you're on the master branch, then HEAD will be pointing to master, and when you commit, that new commit will be a descendant of the revision that your master pointed to, and master will be updated to point to the new commit.
Index: The git "index" is where you place files you want commit to the git repository.The index is a staging area where the new commit is prepared. Essentially, the contents of the index are what will go into the new commit (though if you do git commit -a, this will automatically add all changes to files that Git knows about to the index before committing, so it will commit the current contents of your working tree). The command git add will add or update files from the working tree into your index.