Before the Roman Empire spread to the British Isles, there really was no English language, only local dialects and influences from nomadic tribes—Anglos, Jutes, Saxons, Celts, etc. With the Roman conquest of these regions and the habitation of Roman soldiers, the Latin language merged with the local languages. The history of the English language is very complex, but Latin provided such features as prefixes and suffixes, tense, and conjugation. Today, virtually half of all English words have a Latin root, and the cognates to the Latin language make “latinated” languages easy to learn. In addition, as the English language developed through the Middle Ages and Elizabeth's reign, through such innovators as Chaucer and Shakespeare, Latin became the required language for public office, trade, and learning. In fact, a person could escape the death penalty if he could read the Latin Bible, because it was proof that the criminal was educated and therefore more valuable to the society alive than dead.
Latin influenced English primarily through Norman French, which is a Romance language (descended from Latin).
In 1066 William the Conqueror, Duke of Normandy invaded and became King of England. Before this, Anglo-Saxon English was the common language of England. Anglo-Saxon was a much purer Germanic language. William's language, Norman French, became the language of the English court and the English ruling class and literati. Anglo-Saxon English remained the language of the lower classes. Over time, Norman French influenced the English language and eventually became Middle English. Because of this there are often two words for many concepts in English, one being of Anglo-Saxon origin, the othe of Norman French origin. Examples are, kingly (Saxon) and royal (Norman), or buy/purchase, forgive/pardon, brotherly/fraternal, cow/beef, chicken/poultry.
Another significant influence of Latin came from mideval and Renaissance literature, which was often in Latin. Many concepts which did not have equivelents in English, were simply borrowed from the Latin giving us words like encyclopedia, capacity, dexterity, maturity etc.