Used For :
SharePoint Foundation 2010: Is is for smaller organizations or departments looking for a low-cost entry-level or pilot solution for secure, Web-based collaboration.
SharePoint Server 2010: SharePoint Server 2010 builds on the Microsoft SharePoint Foundation 2010 infrastructure to provide a true enterprise portal platform.Any features that are available in SharePoint Foundation 2010 are also available in SharePoint Server 2010.
SharePoint Foundation 2010: Coordinate schedules, organize documents, and participate in discussions through team workspaces, blogs, wikis, and document libraries on the platform that is the underlying infrastructure for SharePoint Server.
SharePoint Server 2010: In addition to all the Features in SharePoint Foundation 2010, the server includes some additional features for every component available in Foundation 2010. Like Along with the BCS, sharePoint 2010 server also includes External data in search, Secure Store service,External Data Web Parts,Profile pages,External data in workflow,Rich client integration and lot more...