The centre of London is near to Charing Cross station and Trafalgar Square.
For public travel purposes it is divided into concentric zones. If you are coming for a holiday, unless you have a very specific place to go, you will probably rarely venture out of zones 1 or 2, the most central zones.
Greater London, just ‘London’ from now on OK, is divided into boroughs with very distinct personalities, as you will see. You can see them on the first picture. Historic places like Windsor and Hampton Court are some way out of central London.
We are going to keep referring to Central London as a kind of vague term for anywhere you may want to visit, the city, the west end, museums, south bank, whatever…