Drupal's native URL-rewriting functionality is pretty groovy; with a stock install of Drupal you can simply write the custom, semantic/human-readable URL for any node when creating/editing it!

However, things can get a little messy if you plan on writing URLs each time you add new nodes to your site; its makes sense to apply rules to how the URLs are written; such as making all blog entries follow a pattern like 'site.com/blog/[date]/[nodetitle]' - in case you're not familiar with it, there's a module which can do this really simply; its called Pathauto.

I use the Pathauto module on every site that we build in Drupal at Design Guru, but and in keeping up with the latest releases have noticed a change in workflow with the module I'd like to share with you - up until recently you could change the URL role for, say, a node type, and then simply click a 'bulk update' checkmark at '/admin/settings/pathauto' and that would over-write any existing URL aliases you've created on your site.  Great feature eh?  Well, supposedly there were some hiccups with large sites so the module developers have removed this check...

If you're using the newest verison of Pathauto and want to rewrite a rule for a node type etc, you'll either have to edit your database tables to delete the appropriate URLs, then change the rule and update node paths for non-existing aliases (the only option now available in the module) or delete all aliases (easily done at 'admin/build/path') on your site and then chage the appropriate rules, and create new aliases for all nodes using the Pathauto module settings.