In formal language theory, a context-free grammar (CFG) is a grammar in which every production rule is of the form
V → w
where V is a single nonterminal symbol, and w is a string of terminals and/or nonterminals (possibly empty). The term "context-free" expresses the fact that nonterminals can be rewritten without regard to the context in which they occur. A formal language is context-free if some context-free grammar generates it.
Context-free grammars play a central role in the description and design of programming languages and compilers. They are also used for analyzing the syntax of natural languages.
Reference:
http://en.wikipedia.org/wiki/Context_free_grammar
