The working tree/working directory/workspace is the directory tree of (source) files that you are able to see and edit.
The index/staging area is a single, large, binary file in <baseOfRepo>/.git/index, which lists all files in the current branch, their SHA-1 checksums, timestamps, and the file name – it is not another directory which contains a copy of files in it.
HEAD is used to refer to the last commit in the currently checked-out branch.