What is airline code sharing?

We see the term code sharing when applied to flights, what does it mean?
If airline A operates a flight on a certain route, then another airline, say airline B can sell seats on the same flight under a code sharing arrangement. Airline B will usually issue its own flight number and sometimes may not declare that it is not the one actually operating the flight. So you may see on the jetnav flight results, or on perhaps an airport departure board two apparently identical flight timings to the same destination, but there will be only one physical aeroplane.
Why do airlines do it?
Quite simply they are able to offer a wider range of destinations without having to commit the physical resources. It can suit both large and small airlines. The large airline perhaps operates an intercontinental flight to a major airport. A smaller airline based at that airport can then offer a range of flights that tie in with the intercontinental flight. The larger airline can fill its intercontinental flight and gain some revenue from the seats sold on the smaller airlines flights, the smaller airline uses the passengers from the intercontinental flight to fill its flights to the passengers final destinations. It can work well if the airlines choose their partners carefully and there isn't too much route overlap. This is the basis of the major alliances such as Star Alliance and Oneworld. On the jetnav website we have used an asterisk * to indicate which airline is the route operator and which is the code sharer.