There's an excellent series of articles on the RYA website, written by Simon Jinks, RYA instructor on the phenomenon that occurs when out cruising and another boat is heading in the same direction. Jinks starts with the comment, "Many cruiser sailors do not race - until a competitor comes along...."
The eight part series covers how to get involved in local club racing, handicapping or rating, the Racing Rules of Sailing, yachts and equipment, startline rules, signals and tactics.
For the full series, go to the RYA website, Cruiser Racing part 1
(with thanks to Charles Whittam for finding these articles and pointing them out)