Sorry, category error. I was using index to refer to the category of which index is a member. Maybe areas is better. These are the 4 ... things...:
working directory - this is the project directory in the OS file structure
index - a.k.a the staging area
repository - in the .git directory
stash - a kind of scratch pad or clipboard for the developer
Understanding these different areas and how and why to move data into each is essential to understanding git