What is SharePoint?

Microsoft SharePoint, also known as Microsoft SharePoint Products and Technologies, is a collection of products and software elements that includes, among a growing selection of components, web browser based collaboration functions, process management modules, search modules and a document-management platform. SharePoint can be used to host web sites that access shared workspaces, information stores and documents, as well as host defined applications such as wikis and blogs. All users can manipulate proprietary controls called "web parts" or interact with pieces of content such as lists and document libraries.

Windows SharePoint Services 2.0 (WSS 2.0)[v2]
Microsoft Sharepoint Portal Server 2003 [v2]
Windows SharePoint Services 3.0 (WSS 3.0)[v3]
Search Server 2008 [v3]
Microsoft Office SharePoint Server 2007 MOSS [v3]
Forms Server 2007
Microsoft Office Groove Server 2007
Microsoft Office Project Server 2007
Microsoft SharePoint 2010 (currently in Beta)[v4]

Microsoft SharePoint 2010
Described by Microsoft as the "Business Collaboration Platform for the Enterprise and the Web", Microsoft SharePoint 2010 promises to deliver numerous benefits over its predecessor. It is currently in Beta.

Feature highlights include :

New user interface, including the new Office Ribbon.
Web Edit, allowing easy customization of a site.
Silverlight Web Part, allowing rapid integration of rich Silverlight applications.
Rich Theming, allowing simplified skinning of a SharePoint 2010 site
Multiple browser support, including Internet Explorer, Firefox and Safari amongst others.
Enterprise Metadata Management ("SharePoint taxonomy"), which enables a centralized taxonomy
There are also a collection of other new features relevant to IT Professionals, Developers and Partners

Ref: http://en.wikipedia.org/wiki/Microsoft_SharePoint

Microsoft Office SharePoint Server 2007 is an integrated suite of server capabilities that can help improve organizational effectiveness by providing comprehensive content management and enterprise search, accelerating shared business processes, and facilitating information-sharing across boundaries for better business insight. Additionally, this collaboration and content management server provides IT professionals and developers with the platform and tools they need for server administration, application extensibility, and interoperability.

Microsoft Office SharePoint Server 2007 Capabilities
Microsoft Office SharePoint Server 2007 provides a single, integrated location where employees can efficiently collaborate with team members, find organizational resources, search for experts and corporate information, manage content and workflow, and leverage business insight to make better-informed decisions.

Collaboration
Allow teams to work together effectively, collaborate on and publish documents, maintain task lists, implement workflows, and share information through the use of wikis and blogs.

Portals
Create a personal MySite portal to share information with others and personalize the user experience and content of an enterprise Web site based on the user’s profile.

Enterprise Search
Quickly and easily find people, expertise, and content in business applications.

Enterprise Content Management
Create and manage documents, records, and Web content.

Business Process and Forms
Create workflows and electronic forms to automate and streamline your business processes.

Business Intelligence
Allow information workers to easily access critical business information, analyze and view data, and publish reports to make better-informed decisions.

Ref: http://sharepoint.microsoft.com/Pages/Default.aspx
http://sharepoint.microsoft.com/product/details/Pages/default.aspx