You can learn more about user profiles by starting with these resources:
TechNet's Manage user profiles web page provides an IT pro, task-focused introduction to user profiles. From there, the table of contents pane will guide you to additional related articles.
The MSDN articles "What's New for Developers in Office SharePoint Server 2007" and "Personalizing Your Portal" are great resources for developers. I also recommend that IT pros read these two articles and the related articles in the table of contents. MSDN documentation is often useful because it gives an IT pro a glimpse "under the covers," making the "above the covers" more understandable. It also helps both IT pros and business decision makers aware of what can be done by extending SharePoint with custom code.
Office Online offers information about user profiles in "Managing User Profiles from Active Directory". However, although this discussion is under the header of Microsoft Office SharePoint Server (MOSS), it deals with Microsoft SharePoint Portal Server 2003. I'm not convinced that this content is 100 percent on target for MOSS 2007, so I'd use it only as a last resort.
You can find even more resources on user profiles using your favorite search engine. Just use the search phrase SharePoint "user profiles" (include the quotes).