A - Alpha
B - Bravo
C - Charlie
D - Delta
E - Echo
F - Foxtrot
G - Golf
H - Hotel
I - India
J - Juliett
K - Kilo
L - Lima
M - Mike
N - November
O - Oscar
P - Papa
Q - Quebec
R - Romeo
S - Sierra
T - Tango
U - Uniform
V - Victor
W - Whiskey
X - X-ray
Y - Yankee
Z - Zulu

The official name for this is International Radiotelephony Spelling Alphabet and it is also known as ICAO Phonetic Alphabet.
You will need to know it when you want to use Vatsim.
The ingame ATC also uses it for callsigns, taxiways, atis info code and some other instances.
There once was a bug where Ronald Reagan International became:
Romeo Oscar November Alpha Lima Delta Romeo Echo Alpha Golf Alpha November India November Tango Echo Romeo November Alpha Tango India Oscal November Alpha Lima
(glad that's fixed :D)
Hotel Foxtrot Golf Lima