At its very core, SharePoint is a distributed, multi-tiered, Microsoft Internet Information Server (IIS) application, comprising unmanaged, COM-based ISAPI extensions (OWSSRV.DLL being the most important) and APIs; .NET Framework-based APIs hosted in ASP.NET, XML, HTML, CSS, JavaScript, and image files stored in the Windows filesystem; and configuration and content stored in Microsoft SQL Server. Though much of SharePoint's core capabilities are implemented in native/unmanaged COM objects, and though calls to those COM objects are allowed (either directly or through HTTP RPC calls to the ISAPI extensions), all of those capabilities are also exposed by the .NET Framework-based "wrapper" APIs, which include SOAP and REST web services.

SharePoint is a very complex system, and its architecture has many layers of abstraction above IIS, COM, ASP.NET, and SQL Server. Luckily, understanding COM is not necessary for understanding SharePoint, but having a good knowledge of IIS...