Syntax, or grammar, is concerned with the structure or form each utterance from a language can take, and how it is constructed from smaller parts. The two steps in syntactics are that of determining the individual units in each particular sentence, and then finding general rules for how those parts are constructed into sentences. These rules are collected together to form a grammar.
In building a natural language application the type of grammars and syntax designers are concerned with is that of descriptive syntax. So called because it describes the language that people actually use in everyday conversation.
Grammatically correct English is rarely used in spoken language and so prescriptive grammars, those that formally describe the language, are of little practical use in natural language applications as their idea is to provide a natural and intuitive way of interacting with the computer, and forcing users to use language which is not natural to them undermines the aim.
The following sections describe three of the most well known grammar formalisms along with their advantages and their drawbacks.