Questions with this tag should be about both elementary OS and Ubuntu - a Linux distribution with an emphasis on ease of use and installation. Note that http://askubuntu.com is specifically dedicated to Ubuntu questions. elementary OS releases are based on Ubuntu LTS releases normally.
The operating system on which elementary is based.
Ubuntu specific questions should be asked on Ask Ubuntu. This tag can be used for questions that cover both elementary and Ubuntu, but are more specifically about elementary and/or its components.
Ubuntu uses the Unity Desktop Environment, whereas elementary uses its own Pantheon Desktop Environment.
For more info see The Ubuntu Homepage.