Synchronic and Diachronic Linguistics

Etymologically, linguistics is the scientific study of language; but it is not the study of one particular language but of human language in general.

It studies language as a universal and recognizable part of human behavior. 

It attempts to define and analyze language, its place in human life, the way it is organized to fulfill the needs it serves and the functions it performs. Therefore, linguistics studies the origin, organization, nature, scope and development of language descriptively, historically, comparatively and formulates general rules related to language.

Diachronic (Historical) Linguistics studies the development of language through history, through time e.g. how French and Italian have evolved from Latin.
Synchronic Linguistics studies how the people speak and use language in a given speech community at a given time.

In Comparative linguistics, one is concerned with comparing two or more different languages so linguistics is a science, which describes and classifies languages. The linguist identifies and describes the units and patterns of the sound system, the words and morphemes and the phrases and sentences.