I think the problem is that the guide doesn't explain the vocabulary it uses at all. There are programmers (and definitely non-programmers) who might not have heard the word "tree" used to describe a directory filled with files and other directories. It's immediately clear once it's explained, and the guide even has little drawings that look like a directory tree, but for people new to version control and people who have used CVS or SVN this use of the word might be foreign. The same with "commit" since someone coming from another system has a different idea of what that means and someone new to version control has no idea.
Honestly, I'm not even sure you need to talk about "trees". The first chapter of Pro Git doesn't mention them at all, and this section describes the working dir, the index and HEAD without using the word: http://progit.org/book/ch1-3.html Instead he talks about "snapshots" and "stages".
I found it pretty helpful with the use of the word "tree" (got it from http://progit.org/). But I definitely need to add some kind of glossary to the page.
Honestly, I'm not even sure you need to talk about "trees". The first chapter of Pro Git doesn't mention them at all, and this section describes the working dir, the index and HEAD without using the word: http://progit.org/book/ch1-3.html Instead he talks about "snapshots" and "stages".