This blog post is intended for someone who knows very little about SharePoint and has to quickly learn what it is.
Said simply SharePoint is Microsoft tool for creating Internet and Intranet sites. It accelerates the process of creating web pages by giving you a set of pre-created tools to make web sites.
Standard web sites include Portals, blogs, wikis, social networks and team sites.
SharePoints primary use is for document management. With sharepoint you can create a web page for sharing and creating word, excel, and power point documents. This gives you a more controlled and user friendly environment to make and share documents than a shared drive. SharePoint is very useful for project teams which need to work together, often over a wide area.
SharePoint also provides MySites, which can be thought of as a kind of Facebook for businesses. Be warned that only the 2010 MySites is really any good.
As for releases. SharePoint was first released in 2001. The first version that was fairly widely used was released in 2003. The product really took off with a 2007 relases (release 3) and we now have a 2010 release that promises further improvement.
SharePoint comes in a limited feature free download from Microsoft called WSS. WSS 2.0 (2003) and WSS 3.0 are free downloads you can search on Microsoft's site. WSS will be called SharePoint Foundations in 2010.
Microsoft also sells a licensed more feature rich version. SharePoint is almost always licensed on a per server basis, so for very large installation it can get very expensive.
Okay the quick and dirty. SharePoint is a very popular tool for groups and teams to share information. Its kind of a shared drive replacement in a browser. You can also use it to create web pages but be warned: it does not make very clean HTML and if you need a accessible site you might want to shy away.
SharePoint can be greatly extended by buying third party products or writing code in ASP.NET. Popular companies for extensions are Quest, AvePoint, K2, and Bamboo. I am sure I missed some but this is just an introduction.