Read the following text and study the notes below:
The way we are using the word grammar differs in another way from its most common meaning. In our sense, the grammar includes everything speakers know about their language - the sound system, called phonology, the system of meanings, called semantics, the rules of word formation, called morphology, and the rules of sentence formation, called syntax. It also of course includes the vocabulary of words - the dictionary or lexicon. Many people think of the grammar of a language as referring solely to the syntactic rules. This latter sense is what students usually mean when they talk about their class in "English grammar.".
(From An introduction to language by Victoria Fromkin and Robert Rodman (Holt-Saunders))