Deep Dive Into the SharePoint Content Deployment and Migration API
Stefan Goßner posted a great series of posts about the Content Migration API (formerly known as PRIME) for SharePoint (WSS 3.0 and MOSS 2007). In just 5 post he explains everything about this API and where SharePoint uses it. It's an excellent start point for many SharePoint administrators and developers out there. I also refer to this post in my training's about SharePoint as well.
If you want some other great info about migration check Replication and High Availability from Joel Oleson.