The first thing to know about grammar is what it is. In the popular sense, it can mean any study of language, including morphology, syntax, usage, semantics and phonology.
Syntax is the study of the patterns of sentences and phrases from words in a language.
Semantics is the study of meaning, particularly that meaning which can be derived from phrases and sentences in a language. Obviously then, the popular meaning of grammar is too broad, covering far too many fields and is therefore meaningless. Grammar is actually the study of morphology and syntax. The second thing to know about grammar is that at least three theories exist to describe it. It is Latinate because it is based upon rules for inflection and syntax in Latin.