Each human language is a complex of knowledge and abilities enabling speakers of the language to communicate with each other, to express ideas, hypotheses, emotions, desires, and all the other things that need expressing. Linguistics is the study of these knowledge systems in all their aspects: how is such a knowledge system structured, how is it acquired, how is it used in the production and comprehension of messages, how does it change over time? Linguists consequently are concerned with a number of particular questions about the nature of language. What properties do all human languages have in common? How do languages differ, and to what extent are the differences systematic, i.e. can we find patterns in the differences? How do children acquire such complete knowledge of a language in such a short time? What are the ways in which languages can change over time, and are there limitations to how languages change? What is the nature of the cognitive processes that come into play when we produce and understand language?

The part of linguistics that is concerned with the structure of language is divided into a number of subfields:

Phonetics is the study of the sounds of speech. It includes understanding how sounds are made using the mouth, nose, teeth and tongue, and also understanding how the ear hears those sounds and can tell them apart. A study of phonetics involves practicing producing (sometimes exotic) sounds, and figuring out which sound you heard. The wave form of each sound can be analysed with the help of computer programs. In sign language, phonetics refers to the the possible shapes, movements and use of physical space.

Phonology makes use of the phonetics in order to see how sounds or signs are arranged in a system for each language. In phonology, it matters whether sounds are contrastive or not, that is, whether substituting one sound for another gives a different, or "contrastive," meaning. For example in English, [r] and [l] are two different sounds - and the words "road" and "load" differ according to which of these sounds is used. But in some languages, [r] and [l] are variations of the same sound. They could never make a meaning difference in words that differ by only that sound. Phonologists describe the contrastive consonants and vowels in a language, and how pronunciation is affected by the position of the sound in the word and the sounds that are nearby. They are also interested in syllables, phrases, rhythm, tone, and intonation.

Morphology looks at how individual words are formed from smaller chunks of meaningful units called morphemes. For example, the English word 'untied' is really made up of three parts, one refering to the process of reversing an action (un-), one indicating the action of twisting stringlike things together so they stay (tie), and the last indicating that the action happened in the past (-d). Many languages have a much more complex way of putting words together. Morphology interacts in important ways with both phonology (bringing sounds together can cause them to change) and syntax, which needs to pay attention to the form of a word when it combines it with other words.

Syntax is the study of how phrases, clauses and sentences are constructed and combined in particular languages. Writing a grammar requires defining the rules that govern the structure of the sentences of the language. Such rules involve both the order of words, and the form of words in their various possible positions. There are common patterns among even unrelated languages, and many linguists believe this is the result of general principles which apply to most, if not all, languages. For example, languages where the direct object generally follows the verb have a lot of things in common, in contrast to the things in common held by languages in which the direct object generally precedes the verb.

Semantics is the study of meaning. It focuses on the relation between words, phrases and other bits of language and on how these words and phrases connect to the world.

Pragmatics is similar, but it involves the study of how speakers of a language use the language to communicate and accomplish what they want. Pragmatics looks more at the relationship between speaker and listener which allows assumptions to be made about the intended message, considering, for example, the way context contributes to meaning. A classic example is where someone is asked "Do you want some coffee?" Does the reply "Coffee will keep me awake" mean yes or no? It depends whether the person wants to stay awake - and the questioner will only understand the intended meaning if they know whether the person wants to stay awake.

Aside from language structure, other perspectives on language are represented in specialized or interdisciplinary branches:
Historical Linguistics
Sociolinguistics
Psycholinguistics
Ethnolinguistics (or Anthropological Linguistics)
Dialectology
Computational Linguistics
Neurolinguistics
Because language is such a central feature of being a human, Linguistics has intellectual connections and overlaps with many other disciplines in the humanities, the social sciences, and the natural sciences. Some of the closest connections are with Philosophy, Literature, Language Pedagogy, Psychology, Sociology, Physics (acoustics), Biology (anatomy, neuroscience), Computer Science, Computer Engineering, Health Sciences (Aphasia, Speech Therapy).
The main purpose of the study of Linguistics in an academic environment is the advancement of knowledge. However, because of the centrality of language in human interaction and behavior, the knowledge gained through the study of linguistics has many practical consequences and uses. Graduates of undergraduate and graduate programs in Linguistics apply their training in many diverse areas, including language pedagogy, speech pathology, speech synthesis, natural language interfaces, search engines, machine translation, forensics, naming, and of course all forms of writing, editing, and publishing. Perhaps the most widely appreciated application was contributed by UCSC Linguistics alumnus Marc Okrand, who invented the Klingon language for Star Trek.

Wow, that's serious post right there :v
But nope, I just copy & paste from another site.
I don't even read this post 😁

Here's the source if you want to know
 https://www.sil.org/linguistics/what-linguistics
 https://linguistics.ucsc.edu/about/what-is-linguistics.html