ICAO Airport Code

The ICAO airport code is a four-letter alphanumeric code designating each airport around the world. These codes are defined by the ICAO organisation. The ICAO codes are used by Air Traffic Control and airline operations e.g. flight planning. The IATA codes are used for airline timetables/reservations and baggage handling.

Unlike the IATA codes, the ICAO codes have a regional structure, i.e. the first letter is allocated by continent, the second is a country within the continent, the remaining two are used to identify each airport.

Continental letters:
A = Antartica & South Pacific
B = Greenland & Iceland
C = Canada
D = North West Africa
E = Northern Europe
F = Southern & Central Africa
G = West Africa, Canary Islands
H = East Africa
K = USA
L = Southern Europe
M = Central America & Caribbean
N = South Pacific
O = Middle East
P = Hawaii, Alaska & North Pacific
R = East Asia
S = South America
T = Caribbean
U = Russia and former USSR countries
V = South Asia
W = South East Asia
Y = Australia
Z = China

Some country/airport examples:
EBBR: Belgium - BRussels (IATA code BRU)
FAJS: South Africa - Johannesburg Jan Smuts (IATA code JNB)
KBOS: US - Boston (IATA code BOS)
LIRF: Italy - Rome Fiumicino (IATA code FCO)
OMDB: UAE - Dubai

others are less logical, very few UK airports are obvious:
EGLL - London Heathrow, EGKK - London Gatwick (see also List of UK airfields)

A list of airports, sorted by IATA code, is available.