All airports have two codes - the four-letter ICAO code used in flight operations (flight plans, met reports) and the three-letter IATA code used in passenger operations.

ICAO codes always start with the two-letter country code - EG for the UK, EH for the Netherlands, LF for France, CY for Canada etc. Countries with lots of airports have more than one two-letter code - for example Germany is mostly ED but also uses ET, Canada's main code is CY with a small number of airports using CW and CZ. The USA has the whole K series from KA to KZ.

In most countries ICAO and IATA codes are totally unconnected - e.g., Amsterdam is AMS or EHAM, Heathrow is LHR or EGLL, Norwich (my home airport) is NWI or EGSH.

However, Canada has decided to use the same codes for both. Canadian IATA codes are the ICAO codes without the C - CYOW becomes YOW, CYUL becomes YUL etc.