Brazilian Jiu Jitsu is a combat sport that focusses on ground fighting. The sequence of attack usually consists of taking an opponent to the ground and then using specific positions to gain an advantageous position over them.
BJJ is specifically fascinating as it has an open weight category, this means that opponents of any size can take part and a smaller person can (and often does) submit the larger one.
Ground fighting also a crucial part of MMA with a large range of techniques that force an opponent to tap out so this is a great supplementary class to take if wanting to learn MMA.