When one says SharePoint there are Windows SharePoint Service(WSS), Microsoft SharePoint Server (MOSS) 2007,2010 and Microsoft Search Server.
Basically Microsoft SharePoint is a web application platform developed by Microsoft which is typically associated with web content management and document management systems.
Lets take a brief look at each of above.
Windows SharePoint Services (WSS or SPS) is a portal-based Web services on an intranet. WSS is a free download included with every Windows Server 2003 license and is considered to be part of the Office 2003 productivity suite.
Read here for Several "Out of the box" features of WSS.
Microsoft's Office SharePoint Server 2007, or “MOSS” for short, is Microsoft's first integrated server platform that aims to provide web content management, enterprise content services, and enterprise search, as well as shared business processes and business intelligence dashboarding to the small/medium enterprise.
Like its predecessor SharePoint Portal Server (SPS) 2003/WSS 2.0, MOSS is fundamentally dedicated to unstructured document storage, structured list storage, and group collaboration. The word “share” has not been removed from the mission concept, which goes something like “connecting people, processes, and information.”
MOSS is built upon six most important pillars.
1. Collabration
2. Portal
3. Enterprise Search
4. Content Management
5. Business Process and Forms
6. Business Intelligence
-The basic MOSS/SharePoint wheel--
Microsoft's SharePoint marketing refers to the "SharePoint Wheel" to help describe the package of functionality built into the SharePoint platform. The wheel refers to six abstract functional capabilities:
Sites: The SharePoint platform fundamentally enables users to provision 'sites' (public or private) without a requirement for specialized knowledge. SharePoint is designed to become the central location for management of sites in an organization.
Communities: SharePoint aims to support the formation of communities within an organization - these communities may form around teams, projects, clients, geographic locations, etc. SharePoint also provides social features and social integration.
Content: SharePoint provides a central location to put content such as files, documents, or general information. This can be accessed and modified within a web browser or using a client application (typically Microsoft Office) via desktop or smartphone. SharePoint 2010 also provides a concurrent edit ability with Office 2010.
Search: SharePoint provides a range of search abilities, including in documents, in external content (such as network shares or public websites), and in user profiles.
Insights: SharePoint provides data integration, data crawling, and report design to enable business decision making. SharePoint can integrate with SQL Server Reporting Services to surface business intelligence.
Composites: SharePoint provides an application platform based on ASP.NET 3.5 allowing no-code development of complex business problems using SharePoint Designer. SharePoint also allows custom code to be developed using Microsoft Visual Studio 2010.