Your Oxford username is usually of the form abcd1234, where abcd is a code for your college or department. It is administered by IT Services and is separate from any other local college or departmental accounts you may have.
Your Oxford username gives you access to many web-based services at Oxford (e.g. Self-Registration and course booking) through the Single Sign-On service. Single Sign-On means that after your initial login, you can use associated services without having to re-enter your username and password.
It isn't just web-based services that are part of Single-Sign On: you will need your Oxford username to access the central GNU/Linux service. There are also many services hosted outside IT Services that use Single Sign-On such as some library resources and the OxCORT Tutorial system.
You will also need your Oxford username to access your Nexus account via the web interface or via an email client installed on your computer. Strictly speaking the Nexus service is not fully integrated with Oxford's Single Sign-On system and so even if you have logged in to the Nexus web interface you will still need to log in again to access other SSO services; more on this later.
You may hear your Oxford username referred to as your Single Sign-On account (or SSO account), or you may even hear it referred to simply by using the name of a service, e.g. your Nexus account.