A survey article on formal systems that define families of formal languages arising in many computer science applications with primary focus on context-free languages.
An article defining the grammar and how Binary Normal Form (BNF) is used to parse words in a context free language. An example shows how operator precedence is preserved in a context free grammar.
Lecture notes defining context free grammars and closure and decidability properties of context free languages. There is a short section showing that natural languages are not context free.