A concise and simplified definition of linguistics is that linguistics is the scientific study of all forms of language manifestation. It means any form- spoken, writen, or any other one, where language is present.
Linwuistics also deals with other related fields of science. It is for example, language and the teaching of foreign languages, language and literature, language and society, language and music, language and psychology, and a few others. These fields of study are grouped under the term „ applied linguistics“.
Language forms the basis of our interest in linguistic studies. Language is the basic means of human communication. It is certainly the most widely used and most convenient means of human communication. Man cen be referred to as homo lonques, rather than homo sapiens, because it was speech that enabled man to become what he actually is in our time.
Language could become the basic means of communication because language is a convention generally accepted in a speech community. It becomes evident after at the dual nature of linguistic signs :

A linguistic sign consists of a notion , reffering to the nonliguistic world, and its phonetic realization- which is a language phenomenom.The convention lies in the general agreement to use a certain chain of sounds to give an acoustic realization to a notion which exists in our minds. The notion is the same for every language, of course, but its phonetic realization , its phonetic shape, varies from language to language. Here is an example:
If we think of a very common domestic animal that people are familiar with in most parts of the world, the notion will mean the same to people from different languages, but the phonetic realization of it will be very different even between languages which are very close to each other. The phonetic realization of tha same notion of the same animal will be „ dog“ in English, „ hund“ in German, „perro“ in Spanish, „ cachorro“ in Portuguese, „ chien“ in French, „ sobaka“ in Russian, „ kutya“ in Hungarian, „ pes“ in Slovak or Czech.
Regardless of the phonetic shape , the different sequence of sounds in every case refers to the same notion for the people speaking these different languages. It can only be so, because language is a convention.
Clothing, for example, is also a convention,a nd we do not wear the same clothes as Indians or Africans do, because we have agreed upon using different kind of clothes.