An empire is a group of countries ruled over by a single monarch or ruling power. An empire doesn't need an 'emperor'. The British Empire comprised of Britain, the 'mother country', and the colonies, countries ruled to some degree by and from Britain.
In the 16th century Britain began to establish overseas colonies. By 1783, Britain had built a large empire with colonies in America and the West Indies.
Interpretations of the British Empire have changed and developed over time. In the 19th and early 20th century, some historians argued that the empire was the deserved result of Britain's technical and moral superiority. They argued that British rule established formal systems of government, law and education as well as the development of infrastructure, like railways. However, this is a dated view that has been widely challenged. Many modern historians argue that it is unacceptable to say that colonialized peoples did not have or would not have developed their own entirely valid forms of government, laws, and infrastructures without the influence of the British Empire.
Furthermore, many historians argue that you cannot examine the British Empire without examining the more shameful aspects of Britain's past. Britain was heavily involved with the Transatlantic slave trade in the 17th, 18th, and 19th centuries. The British Empire also stripped many colonies and indigenous peoples of their land and vibrant cultures, for example, the Aboriginal in Australia and the indigenous peoples of the United States. In India, colonialisation resulted in the increase in land taxation and lack of reserve crop, which together with poor weather conditions, caused many deaths due to famine, disease and violence. Colonialisation had a similar impact in Kenya in the 1950's.