SharePoint is a collaboration program designed to bring together different participants to share common items. It is a browser based application that can bring together process management modules, search modules and a document-management platform. There are three software elements of SharePoint: Windows Sharepoint Server (WSS), Microsoft Office Sharepoint Services (MOSS), and SharePoint Designer. SharePoint can only be created in a Microsoft Server 2003 or Server 2008 environment.
Windows SharePoint Services (WSS)
This is a free add-on to Microsoft's two Windows Server platforms (2003 and 2008) providing a thoroughly functional web portal. Among its features are the following items: blogs and wikis, discussion boards, alerts (like e-mail), shared calendars and contact lists, collaboration tools, and a content management system.
Microsoft Office SharePoint Server (MOSS)
Office SharePoint Server 2007 supports various Internet operations like extranet, and Web applications for a businesses using one platform, instead of relying on different fragmented systems.

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SharePoint Designer
Office SharePoint Designer 2007 provides an automation tool to identify business processes and build applications like reporting and tracking services, with all of this in an IT-managed environment. The Designer offers customization of the SharePoint site to comply with the business requirements.
Source: Sharepoint
MOSS
Designer