What is a ring main?
A ring main as the name implies, is a wiring ring between a circuit breaker on a consumer unit and the sockets it supplies. 

One wire leaves the consumer unit and connects to the first socket, a second cable then leaves this socket and goes to the next, at the last socket in the chain, the last connection goes back to the consumer unit creating a ring. The power load is shared both ways around the ring allowing use of a smaller diameter cable than a radial wiring system. This smaller diameter was one of the principle reasons for its introduction after the war in order to reduce the amount of copper which was in fairly short supply. 

Older wiring systems have remained in use until fairly recently, my grandparents house was only rewired in 1987 although many were converted earlier.

The wiring scheme has some criticisms including less apparent fault symptoms and more lengthy testing requirements which aside from the complexity of retroactively adopting a new standard have probably contributed to its limited use outside the UK.

The Wikipedia article contains some more details on the standard, its history and issues but adequate training should be arranged before making any modifications to existing circuits.

http://en.m.wikipedia.org/wiki/Ring_circuit 

It's worth noting as well before considering any work that Part P of the Building Regulations may well apply, the IET has a good faq on the subject which is worth reading:

http://www.theiet.org/publishing/wiring-regulations/part-p/