Not offering it as a retail product, Foundation will be available only as an OEM option preinstalled on low-cost server hardware. The low price also includes low functionality, stripping out certain portions of the Server OS found in more pricey versions. One primary limitation is the number of users supported, down to a paltry 15 from Small Business Server 2008's limit of 75.
It's intended only to provide basic functionality, such as file sharing, printer sharing, et cetera. It'll also work with any third-party software that runs on Windows and offers upgrade paths to more expensive versions down the road. Microsoft's answer for low-cost servers may not be elegant, but at least it does help bring the price down.