Nexus is Oxford University's email service.
It is, however, more than a mail service. Through Nexus you can also get access to a calendar, address book, and to-do-list. You can share your Nexus folders (calendar, email, contacts etc) with other Nexus users, for example allowing friends and tutors to see your calendar, either all the detail or just the times you are free or busy.
What you can do and how you do it depends on what you are using to access Nexus. You get the most functionality if you use a Microsoft solution, such as Outlook. Even if you do not, you can still access your email, calendar and use many features through a simple web browser.
You can also use Nexus for collaborative working, for example using SharePoint for sharing documents that everyone in a group can edit.