The Microsoft SharePoint 2010 Software Development Kit (SDK) contains conceptual overviews, programming tasks, samples, and references to guide you in developing solutions based on SharePoint 2010 products and technologies.

You can grab it here: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=f0c9daf3-4c54-45ed-9bde-7b4d83a8f26f

The download contains the SDK for SPF and SPS. No need to download two separate files anymore.

The SharePoint Developer Center can be found here: http://msdn.microsoft.com/en-us/sharepoint/default.aspx