SharePoint is the reason I'm considering taking advantage of a suicide booth.
In all seriousness, the rest of the answers are spot on. The differences between WSS 3.0 and MOSS 2007 commonly trip people up ("why pay for MOSS when WSS is free?" for example). SharePoint is a very complex and rich product that is integrated into other Microsoft applications, like Project Server 2007 and Team Foundation Server.
Why should you care about it? It depends. There are quite a few opportunities out there for experienced SharePoint developers and administrators. It can very quickly become the singular focus of your career if you decide to put a lot of effort into learning it.
