Domain Specific Language

Examples
When is it good to use a DSL?
Internal vs External

A programming language specialized to a particular application domain (fields of applications). HTML is an example of a DSL for the domain of web pages. The language is dedecated to:

a particular problem domain
problem representation
solution technique

Examples

SQL, TeX, CSS, XML variants...

When is it good to use a DSL?

Targeting Domain Experts (TeX targets academia, easier than a Java API for typography, Excel macros are powerful without programming knowledge)
Domain Lends Itself to an Idiom, expressed in a simple syntax (think SQL)
You can eliminate Boilerplate

Internal vs External

Internal DSL is embedded in a programming language, and its structure is dependent on the parent language. Easier to implement.

External DSL is a separate language with the particular domain in mind. Gives more syntax flexibility but harder to implement.