A Web search for the word grammar brings up about 171,000,000 hits. Many of the links lead to discussions of “bad grammar.” In popular usage, grammar can mean anything from misspelling a word to putting an apostrophe where it doesn’t belong.

In Modern English Usage (1926-1964), Fowler defines grammar as “a general term for the science of language.” In the 1965 revision, Gower points out that the science of language is now called linguistics and that grammar is a branch of it.

Linguistics is concerned with such terms as phonology, morphology, accidence, orthoepy, orthography, composition, semantics, syntax, and etymology. Language blogs may attract readers who are interested in all of these aspects of language, but as Fowler points out and Gower echoes, orthography, accidence, and syntax are what most English speakers focus on when they talk about grammar–bad or good.

Orthography: the art of writing words with the proper letters according to standard usage. Misspelled words are errors of orthography. So are misplaced apostrophes.

Accidence: the part of grammar that deals with changes in words to change their meaning, for example, adding endings to verbs or changing their spelling to indicate different tenses (walk/walked, run/ran, go/went), adding letters to nouns or changing their spelling to indicate number (boy/boys, man/men), and spelling pronouns differently to indicate subject or object. “Has went” and “between you and I” are examples of errors of accidence.

Syntax: the arrangement of word forms to show their mutual relations in the sentence.
“Coming out of the auditorium, a purse was lost” is an error of syntax.

Here are Fowler’s and Gower’s simple definitions of the other terms:

phonology: how sounds are made and depicted.
morphology: how words are made.
orthoepy: how words are said.
composition: how words are fused into compounds.
semantics: how words are to be understood.
etymology: how words are derived and formed.