This is a really useful command to have in your toolbox.

For Debian based Linux distros you can easily install it using Apt:
$ sudo aptitude install tree

FYI you can save two key strokes by just typing 'tree'. ;) This will default to your current directory.

Also if your directory structure is very deep and you're just interested in the immediate depth you can pass the -L parameter to limit how deep tree recurses:
$ tree -L 2