2 definitions that explains Domain specific languages .
" Domain-Specific Languages, or DSLs, provide a convenient syntactical means for expressing goals in a given problem domain. For example, SQL provides just enough of a programming language to handle the problems of working with databases, making it a domain-specific language." Source: http://ofps.oreilly.com/titles/9780596155957/RoundingOutTheEssentials.html
I like as well definition from Wikipedia
"A domain-specific language (DSL) is a type of programming language or specification language in software development and domain engineering dedicated to a particular problem domain " Source: http://en.wikipedia.org/wiki/Domain-specific_language