Linguistics is the scientific study of language and language patterns.

A lot of people regard it as part of the humanities, but most linguistic data these days are acquired through pretty rigorous studies.

It mostly focuses on the spoken aspect of language. Field linguistics deals in the study of living or nearly-dead languages. More theoretical linguistics will focus on other fields, which include:

• phonetics, the study of language sounds themselves,

• phonology -- distinct sounds in a language and how they are sometimes interchangeable (e.g. "s" vs. "z" in spoken English),

• morphology -- study of discrete units of meaning (a free morpheme is, for example, the word "house" or "John"; a bound morpheme would be, for example, the plural marker "-s"),

• syntax -- sentence structure, breaking things into phrases and hierarchy (which includes, according to some, the idea of a Universal Grammar, which is a basic set of rules common to every human language)

• and semantics -- word meanings and lexical values.

(do note that field linguistics could and probably does study languages along these lines, but it relates more to recording the legacy of rare, dying languages such as Native American languages, Gaelic(I think?), etc. And I don't know much about field linguistics.)

This means that, for instance, a linguist will not necessarily declare a spoken sentence as "ungrammatical" -- if native speakers would say such a thing, and can understand such a thing, then it is grammatical. This allows for a surprisingly broad range of generalizations in language structure.