SharePoint Foundation is a platform for developing collaborative applications. Licences for this come with Windows Server 2008 (ie: Free if you have Windows Server 2008).
SharePoint Server is an application built on SharePoint Foundation, which adds many powerful enterprise features, such as Web Content Management, Variations and Business Connectivity, amongst others. Licences for this vary with the SKU, and can be very expensive.
SharePoint Designer is a client tool for customising SharePoint environments.
SharePoint Workspace is a client tool to let you take copies of SharePoint content offline, with automatic sync when you are back online.
SharePoint 2010 is designed to be installed on Windows Server 2008 64bit only. You may be able to install SharePoint Foundation 2010 on a 64 bit installation of Windows 7, but this is for small scale evaluation purposes only.
Here is a comparison of the features available between the different version of SharePoint 2010 (also, the first Google Result for 'SharePoint feature comparison'): http://sharepoint.microsoft.com/en-us/buy/Pages/Editions-Comparison.aspx
Here are also the system requirements for SharePoint 2010: