Grammar is the system of a language. People sometimes describe grammar as the "rules" of a language; but in fact no language has rules*. If we use the word "rules", we suggest that somebody created the rules first and then spoke the language, like a new game. But languages did not start like that. Languages started by people making sounds which evolved into words, phrases and sentences. No commonly-spoken language is fixed. All languages change over time. What we call "grammar" is simply a reflection of a language at a particular time.
Do we need to study grammar to learn a language? The short answer is "no". Very many people in the world speak their own, native language without having studied its grammar. Children start to speak before they even know the word "grammar". But if you are serious about learning a foreign language, the long answer is "yes, grammar can help you to learn a language more quickly and more efficiently." It's important to think of grammar as something that can help you, like a friend. When you understand the grammar (or system) of a language, you can understand many things yourself, without having to ask a teacher or look in a book.

Grammar is a field of linguistics that involves all the various things that make up the rules of language. Subfields of linguistics that are considered a part of grammar include syntax, phonetics, morphology, and semantics. Grammar is also used as a term to refer to the prescriptive rules of a given language, which may change over time or be open to debate.

Grammar may be separated into two common broad categories: descriptive and prescriptive. Both views of grammar are in wide use, although in general, linguists tend towards a descriptive approach to grammar, while people teaching a specific language – such as English – might tend towards a more prescriptive approach. Usually, there is a bit of give and take in any approach, with a prescriptivist being at least somewhat descriptive, and a descriptivist having some prescriptivist tendencies.

A descriptive grammar tries to look at the grammar of any spoken language or dialect as it actually exists, judging whether a sentence is grammatical or not based on the rules of the speech group in which it is spoken, rather than an arbitrary set of rules. For example, in many speech communities, a sentence such as, “He done got thrown off the horse,” would be entirely grammatical, and an entire set of rules of grammar can be deduced that explain why that formation is grammatical. In another speech community, however, this sentence might be considered ungrammatical, while a version such as, “Him isa throwned offa horse,” would be the grammatical version. In yet another speech community, both would be considered ungrammatical, with only a version such as, “He was thrown off of the horse,” being considered acceptable.

A prescriptive grammar looks at the norms of speech as given by authoritative sources, such as an upper-class or academic subculture, and creates strict rules by which all speech within that language must abide to be considered grammatical. Few linguists take a prescriptive approach to grammar in the modern age, preferring to describe language as it exists in a given speech community. Many teachers, grammar mavens, and pedagogues in general still have a prescriptive approach towards grammar, however, holding to standardized rules as being the only proper way to speak.

Prescriptive grammar is also used to some extent in teaching a language to non-native speakers. When teaching English, for example, it can be useful to employ a “standard” form of English as a baseline to teach from, to help reduce confusion among students. Once the language has been acquired, of course, a less-prescriptive approach will necessarily take over, as the non-native speaker learns regional rules and new dialects that may not conform to the prescriptive grammar he or she originally learned.