I believe SourceLink is a customer support application that has some of the same features that SharePoint provides, but is focused on a much more narrow market (supporting customers). SharePoint could be configured and customized to do a lot of what SourceLink does out of the box, but I don’t believe SourceLink would be considered a competitor to SharePoint.

If you are completely new to SharePoint, here’s a good, short video that explains the basics of what it does: http://blogs.msdn.com/sharepointbi/archive/2009/08/14/sharepoint-in-plain-english.aspx.