Linguistics is the scientific study of human language. Linguists are interested in learning how language works. Since this knowledge is unconscious, linguists have to observe and then analyze how people speak a particular language.
Language is so basic to what people do and are that it permeates everything in our experience.
And yet, languages can differ from each other in a thousand interesting ways. It is related to certain branches of philosophy. Both are clearly related to the study of history. It is closely related to sociology. It is closely related to psychology.