A Skiff in a sailing context is a high performance competitive sailing craft.
There are a number of different craft which are called skiffs but they are generally small coastal or river boats with a one-person or small crew. Skiffs were originally powered by oars or sails, and sailing skiffs have developed into high performance competitive classes.
In Sydney, Australia and in New Zealand the term is used for a number of racing classes (sizes from 6 ft to 23 ft have existed). These were originally heavily crewed and canvassed boats that were relatively short for the canvas and crew carried and were developed from working boats of the time. This style of boat is still active in the form of Historical 10 foot and 18 foot classes. The Skiff classes developed to become much lighter and faster with relatively smaller (but still very large by any other standards) rigs and smaller crews. 12ft Skiff, 16ft Skiff and 18ft Skiff classes are raced in that form. With two crew on the 12 footer and three on the 16 and 18 these are still heavily crewed boats for their size.
Because the modern 18s have such a high profile the term skiff is widely used internationally to refer to other high performance sailing dinghy classes, mostly featuring asymmetrical spinnaker and trapeze which have been strongly influenced by modern skiffs. Examples include: Cherub Skiff, International 14, 29er and 49er. These boats tend to be less heavily crewed in relation to their length than the traditional Australian Skiff Classes. The term is even used for some single handed boats like the Musto Performance Skiff which are far removed from the heavily crewed original boats.