The premise of BJJ is that most of the advantage of a larger, stronger opponent comes from superior reach and more powerful strikes, both of which are somewhat negated when grappling on the ground. The BJJ style includes effective use of the guard position to defend oneself from the bottom position, and passing the guard to dominate from top position with side control, mount, and back mount positions. This system of maneuvering and manipulation can be likened to a form of chess when performed by two experienced practitioners. A submission hold would be the equivalent of checkmate.
BJJ permits all the techniques that judo allows to take the fight to the ground. Once on the ground the opponents can continue to fight, and winning is usually the result of one participant "tapping out". In sport competition, matches are timed and can also be won by outscoring your oppenent by gaining dominent positions, and completing certain manuevers.
Popular techniques employed by practioners are the triangle choke, kimura, cross-choke, armbar, foot-locks, sweeps, and maintaining dominent positions. Fighting generally begins from the stand-up position and the fighters will work to take eachother down by employing a range of throws, sweeps, and other takedowns. Once on the ground, each oppenent will move to try and gain an advantageous position and apply techniques designed to force the oppenent into submission (tap out).
Muay Thai boxing involves combatents fighting in a stand-up fasion. The use of hand strikes, knee strikes, elbow strikes, and shin(leg) strikes is allowed. Fighters may also clench in an attempt to control their oppenent and land even more devastating strikes.
Thai boxers typically use the shin of thier leg because it is stronger than the foot. The shin is also used to block low leg kicks as well. Shin conditioning is a large part of Muay Thai training. There are many myths about how to properly execute shin conditioning. The most preferred method is through repeated kicking drills using a heavy bag. Please consult a professional trainer before attempting to condition your shins for fighting.
Thai boxing strikes are extremely powerful. One of the most effective strikes is the kick. The two most common kicks used by Thai boxers are known as the teep (literally "foot jab,"), and the TAE(kick)chieng (kicking upwards in the shape of a triangle cutting under the arm and ribs) or angle kick. The angle kick uses a rotational movement of the entire body and is somewhat similar to a karate roundhouse kick, but lacks the rotation of the lower leg from the knee used in other striking martial arts like Karate or Taekwondo. The angle kick draws its power entirely from the rotational movement of the body. Many practioners use a counter rotation of the arms to intensify the power of this kick.
Another powerful kick employed by fighters is the leg kick. This is a short and fast kick aimed at either the inner or outer portion of your oppenents leg. This kick is snapped and is very powerful. Against an unconditioned attacker, it is very plausible that one effective leg kick will persuade them from continuing their attack. Over the course of sport fighting, a correctly delivered series of leg kicks will stall your oppenent's ability to move and in many occasions will end the fight.
** Conclusion **
Both are great styles to become a good fighter. A mixture of both will make you a well rounded MMA fighter.
