Descriptive Grammar: Descriptive grammar is what speakers say, and when, why and how they say it (and not whether they should or shouldn't say it.)
Linguists concern themselves with discovering what speakers know about a language and describing that knowledge objectively. They devise rules of descriptive grammar.
Grammatical or ungrammatical?: "Note too that when a sentence is deemed ungrammatical, it might still be used in certain circumstances. There are special constructions, for example, in which English speakers use transitive verbs intransitively, as when a parent says to a child Justin bites--I don't want you to bite. . . .Calling a sentence ungrammatical means that it sounds odd 'all things being equal'--that is, in a neutral context, under its conventional meaning, and with no special circumstances in force." (Steven Pinker, The Stuff of Thought: Language as a Window Into Human Nature. Viking, 2007)
Acceptability and Grammaticality: Acceptability is the extent to which a sentence allowed by the rules to be grammatical is considered permissible by speakers and hearer; grammaticality is the extent to which a 'string' of language conforms with a set of given rules
"Acceptability . . . is related to speaker's performance, that is the actual use of her language in concrete situations. As stressed by Chomsky, acceptability should not be confused with grammaticality: while an acceptable sentence must be grammatical, not just any grammatical sentence is necessarily acceptable. For a sentence to be judged acceptable, it must also appear natural and appropriate in a given context, be easily understood and, possibly, be to a certain extent conventionalised."