Original: For example, Greater London is sometimes referred to colloquially as "London town". (The "City of London" is the historical nucleus, informally known as the "Square Mile", and is administratively separate from the rest of Greater London, while the City of Westminster is also technically a city and is also a London borough).
Simplified: For example, London is a city, but people often call it "London town" ("the City of London" is a part of London where there are a lot of banks).