http://www.html5rocks.com/en/tutorials/internals/howbrowserswork/#Parser_Lexer_combination
Grammars
Parsing is based on the syntax rules the document obeys: the language or format it was written in. Every format you can parse must have deterministic grammar consisting of vocabulary and syntax rules. It is called a context free grammar. Human languages are not such languages and therefore cannot be parsed with conventional parsing techniques.
Parser–Lexer combination
Parsing can be separated into two sub processes: lexical analysis and syntax analysis.
Lexical analysis is the process of breaking the input into tokens. Tokens are the language vocabulary: the collection of valid building blocks. In human language it will consist of all the words that appear in the dictionary for that language.
Syntax analysis is the applying of the language syntax rules.
Parsers usually divide the work between two components: the lexer (sometimes called tokenizer) that is responsible for breaking the input into valid tokens, and the parser that is responsible for constructing the parse tree by analyzing the document structure according to the language syntax rules.
The lexer knows how to strip irrelevant characters like white spaces and line breaks.
Figure : from source document to parse trees
The parsing process is iterative. The parser will usually ask the lexer for a new token and try to match the token with one of the syntax rules. If a rule is matched, a node corresponding to the token will be added to the parse tree and the parser will ask for another token.
If no rule matches, the parser will store the token internally, and keep asking for tokens until a rule matching all the internally stored tokens is found. If no rule is found then the parser will raise an exception. This means the document was not valid and contained syntax errors.