To get started, install exuberant-ctags. In Ubuntu, this is just "apt-get install exuberant-ctags". Now, from within Vim:
:cd project_rootTo jump to the definition of the symbol under the cursor, use cntl-]. To get back to where you were, use cntl-o.
:!ctags -R .
There's also a taglist plugin for Vim. Once you install that, you can use ":TlistToggle" to open up a window on the left that shows all the things defined in your open files. I have that mapped to "T" by putting the following in my .vimrc: "map T :TlistToggle<CR>".
Thanks to Benjamin Sergeant for helping me get started with ctags.