The belt is called “obi”. The obi indicates the rank of the student.
The systematic use of belt colour to denote rank was introduced by Kano Jigoro, the founder of Judo, who first devised the coloured belt system, and awarded his first black belt in 1886. Initially the system had only two colours; white belts for the novice (unranked) students and black belts for the senior (ranked) students. Levels of progression existed in the form of kyu grades, and eventually a brown belt was introduced into the system to denote students that would soon be ranked.
Karate adopted Judo's ranking system in the early 1920s, and after the introduction of the keikogi (practice clothes) an expanded coloured belt system for the ten novice kyu grades was created.
How to Tie your Belt