A good way to think about SharePoint is as a group of technologies. In this group, the main component is Windows SharePoint Services (WSS). You can't have any other part of SharePoint if you don't have WSS.
WSS is an ASP.NET 2.0 Web application. (ASP.NET builds Web applications, and it's used to customize SharePoint.) When you install WSS, you also have to install ASP.NET, which needs several things to run:
IIS version 6 or 7: Internet Information Services is the Web server that hosts SharePoint
.NET Framework version 2.0 and 3.0: This software set installs ASP.NET and Windows Workflow Foundation (WF). (WF lets users create workflows in applications that are written for Vista, XP and Windows Server 2003 operating systems.)
SQL Server 2000 or later:Microsoft's database management system
and Windows Server 2003 or later: Microsoft's server operating system.
All of the SharePoint technologies use the services that WSS provides, particularly Microsoft Office SharePoint Server (MOSS) 2007. These services include content management, data storage, security, management, services and more. MOSS builds on these services to provide:
core services like business data catalog, Excel services and personalization
application services like dashboards and workflows
people and personalization