All ham radio call signs begin with letters (or numbers) taken from blocks assigned to each country of the world by the ITU - International Telecommunications Union.
ITU is the United Nations agency for information and communication technologies.
Each country's regulating agency will, in turn, allocate a unique call sign to each newly licensed ham radio operator.
An Example From Canada
For example, my call sign VE2DPE was taken from the block of prefixes VAA-VGZ allocated to Canada by the ITU.
My call sign, as every other amateur radio call sign in the world, is composed of three parts:
VE = country Canada
2 = province of Québec.
DPE = my own unique ham radio station identifier in Canada.
Canada's telecommunications regulating agency - Industry Canada - can also pick amateur radio call sign prefixes from the following blocks of letter prefixes: CYA-CZZ, VAA-VGZ, VOA-VOZ, VXA-VYZ, XJA-XOZ, CFA-CKZ.
(Canada's regulating agency used to be Communications Canada, but that's another story).
In The United States of America
In the United States, ham call signs are allocated by the FCC from the following blocks of letters: AAA-ALZ, KAA-KZZ, NAA-NZZ, WAA-WZZ.