Nexus has many advantages over "free" email accounts from companies such as Gmail, Hotmail and Yahoo, including greater reliability, speed, security, a wide variety of access methods and a backup policy. The Nexus account also includes access to other features in addition to email, such as calendaring, address books, and to-do lists.
If you already have an external web-based email account, there is nothing to prevent you from continuing to use this while in Oxford, although (for the reasons given above) you are advised to migrate your email to Nexus. Alternatively, if you continue to use your external account, you could set up your Nexus account to forward to your external one (or vice versa), so that you can be contacted using your "standard" Oxford email address. (Auto-forwarding is configured via the Nexus account settings page)
Any current member of the University can use Nexus. All new staff and students have a Oxford account (also called Single Sign-On or SSO) set up for them automatically following the issue of a new University Card. The account is used to access Nexus and other central University services. Details of the account get sent via the internal post to either your college in the case of undergraduates, or department in the case of postgraduates and staff. If the letter has not arrived within two working days of receiving your University Card, please contact OUCS, e.g. via our Problem Report web page and a replacement set of details will be put in the internal mail. Alternatively, you can visit OUCS in person.
Once you have received the details, new Oxford accounts need to be activated by visiting https://webauth.ox.ac.uk/activate.
Oxford accounts (also known as Single Sign-On or SSO accounts) are increasingly used as the basis for accessing other services such as Weblearn and also the OUCS online self-registration services. Even if you have no need to use the Nexus email facility, you should activate your Nexus/SSO account as you will almost certainly need to use its username and password for other purposes.
As well as providing email, an account on Nexus also provides access to web-space for the setting up of personal web pages. More information on this is available elsewhere.