I'm suprised nobody responded to this yet. I'll try since I trained in Muay Thai for about 4 years. Basically you can expect boxing plus knees and elbows. Which of these 3 are emphasized depends on the gym. There are different types of knee and elbow techniques just like how with punching you have a hook, straight right etc. Elbows are generally not allowed in amateur bouts. There is no fighting on the ground, but you do learn some takedowns, by sweeping the leg and/or throwing them to the ground from a clinch position. In a bout, a takedown would score like a landed strike.
Differences with kickboxing: Muay Thai uses just two types of kicks - roundhouse and "tiip". The tiip kick is a pushing kick to the stomach or hip to create distance. The round kick is thrown so you connect with your shin, not the foot. You basically want to connect with the entire length of the shin, laying it across the other guy's thigh, torso, or head. In my opinion this is much more effective than kickboxing kicks, and I think proof lies in the fact that in kickboxing matches fighters are actually required to put in a quota of kicks every round. Muay Thai guys, especially the Thais themselves, will often throw more kicks than punches.
The biggest difference is probably that in kickboxing, if two guys tie each other up, the ref breaks them up. If this happens in Muay Thai, things are just getting started. There are techniques for how to control the other persons body by grabbing the back of the head, and all sorts of dirty tricks to land knees and elbows in the clinch.
The boxing part is like western boxing with some little tweaks. The stance is more open so you can quickly start a kick with either leg, and block those outside low kicks with your shin. Bending the knees to come in low, typical in boxing, is frowned upon, since it sets you up to get kneed. You're also supposed to hold your hands up higher for blocking elbows and high kicks, as opposed to boxing where you keep your elbows tucked for blocking body shots.
You might have some trouble finding matchups at 100+ kilos in Muay Thai. But the good thing about Muay Thai is that you could probably (at least at the amateur level) hold your own in boxing or kickboxing matches as well, which would give you more opportunities to compete. Good luck if you decide to get into it.