SharePoint 2007

Windows SharePoint Services (WSS) v3.
This is the free/unlicensed version of SharePoint that is built upon Windows Server 2003/2008, SQL Server 2000/2005, IIS, ASP.Net 2.0, and .NET 2.0/3.0. All you need is a licensed copy of Windows Server 2003 or Windows Server 2008.

Microsoft Office SharePoint Server (MOSS) 2007.
This is the licensed version of SharePoint that is built upon WSS v3. It has two primary versions: MOSS 2007 Standard and MOSS 2007 Enterprise.

SharePoint 2010

SharePoint Foundation 2010.
This is the free/unlicensed version of SharePoint. It replaces Windows SharePoint Services (WSS) 2007. SharePoint Foundation is built upon Windows Server 2008, SQL Server 2008, IIS and .NET 3.5. All you need is a licensed copy of Windows Server 2008.

Microsoft SharePoint Server 2010.
This is the licensed version of SharePoint, offering a business collaboration platform for the enterprise.
It replaces Microsoft Office SharePoint Server (MOSS) 2007

SharePoint 2013

SharePoint Foundation 2013.
This is the free/unlicensed version of SharePoint. SharePoint Foundation is built upon Windows Server 2008/2012, SQL Server 2008, IIS and .NET 4.5. All you need is a licensed copy of Windows Server 2008 or 2012.

Microsoft SharePoint Server 2013.
This is the licensed version of SharePoint, offering a business collaboration platform for the enterprise.
It replaces Microsoft SharePoint Server 2010.