"Grammar" is usually taken to be a synonym to "syntax" and here's how syntax is defined in Wikipedia:
In linguistics, syntax (from Ancient Greek word for "arrangement" from the word for "together", and word "táxis", "an ordering") is the study of the principles and rules for constructing phrases and sentences in natural languages.
Syntax is concerned with the structure/mechanics of the sentence, order of the principal parts of the sentence and their mutual dependencies. The highest unit of the analysis is the sentence, and further analysis would include phrases. Internal structure of words is the subject of lexicology, morphology or phonology and is not of primary concern for syntax, at least not in the narrow sense of the word. So, in syntactic/grammar analysis the goal would be to analyze principal parts of the sentence: subject, verb, object.., their order (is it SVO which is typical for English, or ditransitive SVOO etc.), their dependencies, verb complements. You would also analyze the formal structure of the sentence elements identifying word classes in the sentence: nouns, verbs, adjectives etc.., and their relation to the functional parts of the sentence.
When we are talking about "usage", it's what it says on the tin. The matter of concern is typically how a particular phrase/expression stands in the wider context of the language. The question about the usage of the word or a phrase would include analyzing the register of speech in which the expression is used- is it colloquial or academic, formal or informal, characteristic of written or spoken language, literary, slang etc. Another important question about the usage is whether a particular usage is idiomatic or not, that is, if there are variations of the same phrase or it is a set expression.Inquiring about the usage of the expression could further include regional spread or source of the phrase - British or American, Australian, Zealandian, Pidgin, Geordy, etc.., age of the people who characteristically use the phrase: young, old, social class: posh expression versus vulgar etc. The principal thing asked about the usage of the expression would also be how the context affect the use of the expression. Does it carry a negative connotation in specific contexts, or possibly sounds jocular, or pejorative etc.
So the terms "grammar" and "usage" refer to two different analyses, which may or may not be both included in analyzing a particular sentence or expression.