Very good points so far but I'll try my best to add something. :)
SharePoint is not just 2 technologies. It is a set of products and technologies brought together by Microsoft into one immense product that comes in 2 flavors. The 2 flavors are Windows SharePoint Services (WSS) and Microsoft Office SharePoint Server (MOSS). MOSS does come in standard and enterprise.
[Some of the technologies used in SharePoint: Windows Workflow Foundation, ASP.NET, Web Parts, XML (included XPath, XSLT, etc), SQL, Web Services - to name a few I can think of off the top of my head]
No matter the version you choose, SharePoint allows for web-based capabilities to allow users to create, organize, distribute, and maintain information. Because of this, the most common uses for SharePoint sites are intranets and project/team sites.
SharePoint also has incredible possibilities as an application platform. Looking at the web part and workflow pieces alone you can begin to realize the potential. For example, automation of authorization processes within an organization can quickly be developed without any code using SharePoint Designer. (FYI: more complex workflows would require Visual Studio but many simple workflows can be designed using the point and click functionality of SharePoint Designer)
While MOSS only extends upon the WSS, it does add a large amount of functionality that can be very important and useful to a business. Some of the more important features available in MOSS and not in WSS are: records management, document retention and auditing policies, browser based forms (InfoPath forms without installing InfoPath on client machine), and some of the business intelligence capabilities. Amazingly we're seeing interest in the social networking features of MOSS too. (easy to read list of features not in WSS that MOSS has)
Why is SharePoint used? I was doing some research not to long ago on this exact subject and I found a research study that cited 5 key benefits:
- Ease of information access
- Streamlined internal communication
- Increased end-user productivity
- Optimized document management practices
- IT time savings
Sorry if that turned into a bit of a ramble.