WSS 3.0 (Windows Sharepoint Services) has a much smaller set of features than Sharepoint Portal Server 2007 (or as Microsoft sometimes labels it Microsoft Office Sharepoint Server, MOSS)
MOSS has two editions, Standard and Enterprise. We use Enterprise.
We did a 3 month analysis of the Sharepoint search engine versus Google Appliances which included a trip to the Google headquarters to tell them we chose Sharepoint!
Overall I couldn't imagine working without it. There are a number of annoyances, however, we always seem to find a workaround. The main one is that it doesn't provide direct support for File Shares. I've been told that will be fixed in the next release.