A domain specific language is a language that’s written to deal with a specific domain or set of concerns. There are a lot of them around, like make, ant, and rake for describing software builds, or lexx and yacc for language construction. In recent years, they’ve become popular as some things have combined to make them easier to build. Big among those things has been the increasing popularity of Ruby, which has several features that make it easy to build new DSLs.
Martin Fowler is a big proponent of the idea, as here.