"Where in the world?"
Recently a cousin asked what the "N" in my call meant. Callsigns with W, K, N, or A in the prefix indicates that the Amateur Radio Operator is in the United States of America.
The numbers 0 through 9 usually indicate which region of the United States that they live. For example: the "4" region is the South East part of of United States. Until a few years ago, if you moved to another number region you were required by the FCC to change your callsign but this rule is no longer in effect and you may keep any call that you will receive from the FCC or that you may presently have for your lifetime.