Licenses for amaetur radio are issued by government
agencies around the world. Letters, numbers, or a
combination of the two are allocated for each country or
possession. For example, the United states has call
signs starting with the letter "W" or "K". Commercial
television and radio stations also use this system (hence
"WNBC" or "KSFO"). Amateur radio includes a digit (number)
in a call sign that usually (but not always) corresponds to
a region of the country. So, "KZ1O" and "WB1GOF" are
typical call sign forms for a US station. The United States
also has allocations for any call sign beginning with "N"
and some that begin with "A".
Call signs are as personal as names in the amateur radio
world. The FCC recently made provisions for a vanity
call sign program where one could select among any unused
call sign (limited only by the license class of the
applicant). I selected NE1RD for reasons that are
obvious to any who know me well. Other interesting (and
fun) call signs I've seen are AC0W and AB0ZO.