Webauth is the system that underlies Oxford's Single Sign-On (SSO) service. Single Sign-On means that after your initial login, you can use associated services without having to re-enter your username and password. Your SSO username is known as your Oxford username but you may also hear it called your SSO account or Webauth account.
You can read more about Webauth in the context of the Oxford username and SSO.
If you are interested in the technical details of implementing and using Webauth take a look at the Webauth HOWTO.