Formal Languages and Compilers - Course notes onthe definition of formal languages and the use of context free languages in compiler construction. - http://shekel.jct.ac.il/~rafi/formcomp.pdf#search=%22context%20sensitive%20languages%20compilers%22
Formal Grammars and Languages - 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. - http://www.cs.ucr.edu/~jiang/cs215/tao-new.pdf#search=%22context%20sensitive%20languages%20compilers%22
Push-Down Automata and Context-Free Grammars - 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. - http://people.umass.edu/partee/726_04/lectures/Lecture%2014%20PDAs%20and%20CFGs.pdf#search=%22pushdown%20automata%22
Context-Free Grammars and Parsing - 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. - http://eli-project.sourceforge.net/elionline4.4/syntax_1.html
Context-free Grammar - A Wikipedia article that defines context free grammars and uses them to generate context free languages. - http://en.wikipedia.org/wiki/Context-free_grammar