It's roughly equivalent to a UK postcode, but much less detailed so usually a town will have only one code, which is why we often get daft questions on here asking what Londons' zip code is. The basic zip code is a five-digit number, running from 00000 in the east to 99999 in the west (the lowest code currently in use is 00601 in Puerto Rico, the highest is 99901 in Ketchikan, Alaska). Thus New York City has a number of zip codes in the 10000 area, and of course famously Hollywood, California is 90210.

There is a more precise "5+4" zip code, but when people talk about zip codes they usually mean the five-digit number.

A US zipcode is roughly equivalent to a UK postcode and is a 5 digit code.

1492

It is roughly the American equivalent of the UK postcode. You will need a specific address to look up what the zip is for an area in the US.

Nothing to do with London.

There are hundreds of them. It depends on where you are sending mail.