Rewrite rules are one of the features of WordPress I’ve never had to deal with in more than quick and cursory way. For the most part, custom post types and pages are good enough for my WordPress site to have all the URLs I need. But when you do need custom URLs for some other purpose — a custom API, for example — you’ll be using the Rewrite API.
Now, having read Carl’s post about how WordPress and the Rewrite API works, though, I have a good sense that in the cases that I’ll need it I’ll be able to get along fine. It’s quite a long and thorough write up about the whole thing, but that’s just the kind I like best. If you want to be ready, need a good reference, or want to brush up on your rusty skills with custom URL structures in WordPress, check it out!