English is a West Germanic language that originated from the Anglo-Frisian dialects brought to Britain by Germanic settlers from various parts of what is now northwest Germany and the northern Netherlands. The resident population at this time was generally speaking Common Brittonic—the insular variety of continental Celtic, which was influenced by the Roman occupation. This group of languages (Welsh, Cornish, Cumbric) cohabited alongside English into the modern period, but due to their remoteness from the Germanic languages, influence on English was notably limited. However, the degree of influence remains debated, and it has recently been argued that its grammatical influence accounts for the substantial innovations noted between English and the other West Germanic languages.
Initially, Old English was a diverse group of dialects, reflecting the varied origins of the Anglo-Saxon Kingdoms of England. One of these dialects, Late West Saxon, eventually came to dominate. The original Old English language was then influenced by two waves of invasion: the first was by speakers of the Scandinavian branch of the Germanic family, who conquered and colonised parts of Britain in the 8th and 9th centuries; the second was the Normans in the 11th century, who spoke Old Norman and ultimately developed an English variety of this called Anglo-Norman. These two invasions caused English to become "mixed" to some degree (though it was never a truly mixed language in the strictest sense of the word; mixed languages arise from the cohabitation of speakers of different languages, who develop a hybrid tongue for basic communication).
The more idiomatic, concrete and descriptive English is, the more it is from Anglo-Saxon origins. The more intellectual and abstract English is, the more it contains Latin and French influences e.g. swine (like the Germanic schwein) is the animal in the field bred by the occupied Anglo-Saxons and pork (like the French porc) is the animal at the table eaten by the occupying Normans.
Cohabitation with the Scandinavians resulted in a significant grammatical simplification and lexical enrichment of the Anglo-Frisian core of English; the later Norman occupation led to the grafting onto that Germanic core of a more elaborate layer of words from the Romance branch of the European languages. This Norman influence entered English largely through the courts and government. Thus, English developed into a "borrowing" language of great flexibility and with a huge vocabulary.