A mailing list is a convenient way to send messages to a group of people. There are lists for different purposes and groups. You may be added to some lists automatically - such as a list for your college or course - and there are others that you choose to join.
Joining a subject-specific mailing list is a good way to learn about developments, discussions, and news for your area of interest. Mailing lists can also be excellent tools for getting answers to subject-specific questions and for finding people who are working in your area. Many lists keep archives of previous postings, which can be useful for finding information quickly and for avoiding repeating a question that has been discussed before.
Good starting points for finding relevant mailing lists are the webpages of societies and associations. Also explore JISCmail - a national academic mailing list service which hosts a large number of lists. For Oxford-specific lists, see http://help.it.ox.ac.uk/email/maillists/index.