Grammar is the system by which a language functions. The description of that system is also called a grammar. Children study grammar in school to improve understanding of their language, and of other languages.
All languages do not function in the same way. Latin depends heavily on changes in the forms of words. Chinese stresses the pitch of the speaker's voice. English emphasizes the order of words. However, many languages include all these features to some degree.
Many grammarians believe that babies begin to learn basic grammar during their first year. By the age of 4 or 5, children have absorbed enough of it to communicate their thoughts. By the time they start school, they have used grammar continually for several years.
Anyone who understands how a language functions can identify that language even if most of the words are nonsense. For example, the major features of English grammar appear in the nonsense sentence The plomic basinkers pirked the lampix at the simter ciptically.
The above combination of words begins with a capital letter and ends with a period. It also has the rhythm of an English sentence. But more important, the words function like an English sentence. The word basinkers is a noun because it has a plural ending, -s. It also is marked by the article the. In the same way, lampix and simter are nouns because both are marked by the article the. The two words probably are singular nouns because they do not have a plural ending.
The word plomic is probably an adjective because it appears between an article and a noun. Pirked is probably a verb because -ed is a characteristic past tense ending of verbs. The word also has a position in the sentence typical of a verb. Pirked is a transitive verb because it has an object, lampix. At the simter can be identified as a phrase that modifies pirked, telling where. At is a preposition often followed by an object. Ciptically is probably an adverb because it ends in -ly. It may modify pirked, telling how or when.
Not all grammarians describe the system by which language functions in the same way. Various grammarians have formulated their own descriptions of how English grammar functions. The best-known descriptions are called traditional grammar, structural grammar, and generative grammar (also known as transformational grammar). Traditional grammar defines parts of speech by their meaning and function. Structural grammar defines them primarily by their order in a sentence. Generative grammar shifts the emphasis from analysis of parts of speech to the way people produce all of the possible sentences of the language.
