The Jiu Jitsu gi is the uniform worn by practitioners of Jiu Jitsu. In some other martial arts such as Karate and Judo they also have uniforms known as gis. That's because word gi means “clothes” in Japanes (more specifically, the Japanese word ki which means “to wear”). Gis are also sometimes called kimonos, especially in Brazil. A kimono usually refers to the upper jacket and not the pants, but generally can also be used to refer to the whole outfit.
The BJJ gi is similar to the Judogi, but it is not as loose. Brazilian Jiu Jitsu kimonos have tighter cuffs on the jacket and pants. This slimmer fit gives BJJ practitioners less material to work with to manipulate.
If you’re training in traditional Brazilian Jiu Jitsu, you will need to wear a Brazilian Jiu Jitsu gi. Although “no gi” Brazilian Jiu Jitsu has become popular due to the growth of MMA, the wearing of a gi is a requirement in order to be promoted.