Pattern Languages:

Pattern have a context in which they apply. When several related patterns are woven together, they form a pattern language. Rather it is a structured collection of interrelated patterns that provides vocabulary for talking about a particular problem.

Objective and Scope:

By providing specific design and development vocabularies pattern languages help software developers communicate better. They cover particular domains and disciplines, such as concurrency, distribution, organizational design, and many more.
In some pattern languages patterns are structured. structures of pattern languages can take different forms, such as network of trees. For example in the G++ pattern language for design of large software system all the patterns are structured in a tree