Grammar is the way we arrange words to make proper sentences. Word level grammar covers verbs and tenses, nouns, adverbs etc. Sentence level grammar covers phrases, clauses, reported speech etc.
In linguistics, grammar is the set of structural rules governing the composition of clauses, phrases and words in a natural language. The term refers also to the study of such rules and this field includes phonology, morphology and syntax, often complemented by phonetics, semantics and pragmatics