Use this tag in addition to the 'linux' tag for questions where integration in the default Ubuntu interface is a requirement. If you're just looking for Linux software and happen to be running Ubuntu, use only the 'linux' tag.
There is a Stack Exchange site dedicated to Ubuntu which is a good place to ask any Ubuntu-related questions if you have questions which are not asking for software recommendations.
All Linux software runs on any distribution. Therefore most questions asking for software to run on Ubuntu should be tagged with linux and not ubuntu, since answers would be equally useful to people running other distributions. Use the tag ubuntu only for questions that are specific to Ubuntu, e.g. if you're looking for a Unity plug-in.