What hook to use to add data to a CCK node-type when it is updated?

I've manually created a CCK content type (don't know how to do it via a module yet, unfortunately). How do I alter data, or insert extra data as the content is being saved?

Specifically, I want to grab the node's taxonomy data, and save it also within the node itself, so it can be easily accessed by the Views module.

I'd be extremely grateful for help from anyone!