Each amateur radio station (ham radio station) has its own unique "name" allotted by the authorities. In amateur radio, the unique name assigned to a ham radio station and its owner helps you to know its operator as well as the country to which he belongs. This particular ham radio station is authorized to be operated by that particular ham radio operator only. However, other licensed ham radio operators are also eligible to operate his station provided they hold licences  of the same category (and not lower than the category which he holds) and the owner of that particular station is physically present during the operation. A person who does not hold a ham radio licence is not authorized to operate a ham radio station. This means that only a single station with a particular call-sign can exist in a particular location. A person having a ham radio call-sign is not authorized to set up more than one ham radio stations at different locations.

A call-sign consists of a set of letters and numbers, assigned by the authority. For example VU2MSY (I was holding this call-sign for the last 10 years till I upgraded my licence with a new call-sign: VU2MUE) is a call-sign which belongs to  a particular ham in India. No other hams anywhere in the world can have this call-sign! Similarly, 4S7VK is a call-sign which belongs to a ham from Sri Lanka. In a call-sign, the letters before the number is called the prefix. So, there might be a ham with the call-sign 4S7MSY in Sri Lanka. Similarly, a call-sign like VU2VK may exist in India! Each country has its own prefix. The knowledge of prefix helps us identifying the country of origin of a particular amateur radio transmission. Call-sign of all the ham radio operators are available in the International Call-book.

When a ham radio station is set up on board a ship, the call-sign should be followed the suffix /MM. So, VU2MSY would become VU2MSY/MM (maritime mobile) when it operates from a ship. If it operates from a vehicle, then the call-sign becomes VU2MSY/M (mobile).

Address from which the International callbook is available:
International Radio
Amateur Callbook
P.O. Box 2013, Lakewood, NJ 08701 USA