A sampler is an album, CD or cassette, which showcases underexposed artists from either a particular genre or, more usually, a particular record label. They are often distributed in ways other than the normal mass-market CD emporia. For example, they can be given away or sold at gigs, sent as mailshots or distributed to music journalists. When sold, they tend to be priced at less than is normally paid for a CD.
The aim of a sampler is as a traditional loss-leader. If a sampler is sold at a little above cost and, say, 10% of the people who hear that sampler then go on to buy a (full-price) record by one of those artists, the sampler has done its job.
An excellent example of a sampler is the Ninja Cuts compilations, released roughly annually through the 1990s as a collection of sounds from the Ninja Tune label. These have acquired something of a reputation as a cut above other samplers and are priced accordingly. The Osmosis sampler on Leaf records is another excellent sampler; Golfing on the Moon on Moonska yet another.