James Waymire, a Microsoft SharePoint support engineer, has a great recent post on his blog talking about patching SharePoint. In this post he addresses a common question support guys get (not to mention consultants) from customers: What patches do I need to keep my SharePoint [WSS/MOSS] Updated?" While it is quite straightforward to may folks, it isn't to others.
I'd recommend this as a good read for anyone who's a bit fuzzy on the subject:
In addition I'd also recommend you check out Spencer Harbar's post SP1 update list page for links to available hotfixes, how to get them, the order to install them, and what's included in security rollups such as service packs or the recently releases Infrastructure Update.