Practically all languages spoken on earth today can be traced by scholars back to some common source, that is, an ancestor language which has many descendants. The ancestor language -together with all the languages which have developed from it- is called a "family" of languages.
English is considered a member of the Indo-European family of languages. Other languages belonging to the same family are French, Italian, German, Norwegian, and Greek.
In this Indo-European family of languages there are various branches and English is a member of the "West Teutonic" branch. Actually, English dates from about the middle of the fifth century, when invaders from across the North Sea conquered the native Celts and settled on the island now known as England.
For the sake of convenience, the history of the English language is divided in to three great periods: the old English (or Anglo-Saxon), from about 400 to 1100; Middle English, from 1100 to 1500; and Modern English, from 1500 to the present day.
So the original language spoken in English was Celtic. But the Anglo-Saxons (the Angles, the Jutes, and the Saxons) conquered the island so thoroughly that very few Celtic words were kept in the new language.
The Anglo-Saxons themselves spoke several dialects. Later on, the Norsemen invaded England and they introduced a Scandinavian element in to the language. This influence, which was a Germanic language, became a part of the language.
In 1066, William the Conqueror brought over still another influence to the language. He made Norman French the language of his Court. At first, this “Norman" language was spoken only by the upper classes. But gradually its influence spread and a language quite different from the Anglo-Saxon developed. This language became the chief source of modern English.