From the Ubuntu Wiki:
Ubuntu Core is a minimal rootfs for use in the creation of custom images for specific needs. Ubuntu Core strives to create a suitable minimal environment for use in Board Support Packages, constrained or integrated environments, or as the basis for application demonstration images. It is available for the i386, amd64, and armel architectures.
Ubuntu Core delivers a functional user-space environment, with full support for installation of additional software from the Ubuntu repositories, through the use of the apt-get command.
Additional information may also be found at Canonical's Engineering services page for Ubuntu Core.