Dataset gives you an easy, declarative method for constructing sets of data to be used in testing. It is a data loading framework. This is in contrast to Machinist and Factory Girl which focus only on making it easy to create new models for testing, but have no solution for loading sets of data.
Read more: http://wiseheartdesign.com/2008/12/12/h1-meet-dataset-the-fixture-killing-data-loading-framework