In an effort to ensure reliable and understandable communications, the International Telecommunications Union, of which the U.S. is a member, created a standard phonetic alphabet, very similar to the old NATO phonetics.

Why is such a "standard" important?  Because when there is a lot of noise and interference it is often difficult to pick out words and spelling is often necessary.  By using the standard phonetics we can often determine which phonetic is being used by hearing even a fragment of the word. For instance, if I were phonetically spelling a word and you heard ..rav.. rome... ...ndia... alph... ..vember, you could determine I was spelling BRIAN.

On the other hand, using non-standard phonetics can create confusion and inaccuracies.  For instance, the common use of "Kilo-Watt" can be interpreted as the letters K and W.  If picking fragments from a noisy signal, a fragment of Watt would be unidentifiable.  

For every rule there is an exception.  For instance, I'll use standard phonetics for my WX4BK call sign, but will often tell people they can remember it by thinking "Weather for Burger King". I've also used, and recommended the use of, non-standard phonetics on Field Day.  Our club call sign, N4FWD, sounds pretty boring as November Four Foxtrot Whiskey Delta, but captures attention when Whiskey 4 Four-Wheel-Drive is used. However, when called out of the pile-up I revert back to standard phonetics.

A - Alfa
B - Bravo
C - Charlie
D - Delta
E - Echo
F - Foxtrot
G - Golf
H - Hotel
I - India
J - Juliet
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 

Of course, being human, we'll make mistakes.  Sometimes we can't remember the proper phonetic, in which case I recommend using whatever comes to mind (assuming it's suitable to be broadcast over the air).   

However, non-standard phonetics should be used judiciously, as what you do in practice you will do under stress, and EMCOMM can be stressful.  Use "Kilowatt" or "Weather for Burger King" every day, and that's what you'll do during an emergency, or just as bad, hesitate groping for the right phonetics.

Practice makes perfect, so while we'll all have the occasional bit of fun with phonetics, remember they serve an important purpose, and the use of the standard phonetic alphabet should be perfected.