London is the capital city of the United Kingdom. In Metropolitan Police terms, this is roughly (but not strictly) within the M25 motorway (called a Freeway or Highway in other English-speaking countries and usually bearing the prefix 'auto-' in most European countries). In most cases, the M25 is considered the border of London.
London is a city of 8 to 10 million people and located in South-East England. London's borders are not actually very well defined, and there are popular misconceptions (see below). London was occupied since Roman times and there is no evidence to suggest that it wasn't inhabited before that. At first, London was a port city, due to its proximity to the mouth of the River Thames, although the ever increasing size of ships means that most seafaring boats don't make it further than Tower Bridge.
London's borders are very confusing. London 'proper' refers only to the City of London (The 'square mile' of old Roman London), a county in its own right. Many people's understanding of London is 'within the M25', which is not 100% correct. Greater London is the collection of London Boroughs (Sun Hill is in Canley). This is roughly the same as the M25, but not the same. Metropolitan Police jurisdiction covers the same area. The City of London has a separate police force.