Punching techniques (Chok) in Thai kick boxing

The punch techniques in Muay Thai were originally quite simple being crosses and a long (or lazy) circular strike made with a straight (but not locked) arm and landing with the heel of the palm. Cross-fertilization with Western boxing and western martial arts mean the full range of western boxing punches are now used (jab, straight right/cross, hook, uppercut, shovel and corkscrew punches plus overhand or bolo punches).

Muay Thai judges score punching techniques less highly than other strikes as they are generally less powerful than knee strikes or kicks and the fists are padded by gloves (while knees, elbows, shins, and feet are not).

As a tactic, body punching is used less in Muay Thai than other martial arts to avoid exposing the attacker's head to counter strikes from knees or elbows.

From Wikipedia

Muay Thai (Thai kick boxing) techniques

In its original form, Muay Thai consisted of an arsenal of nine weapons - the head, fists, elbows, knees and feet - known collectively as na-wa arwud. Although in modern Muay Thai, both amateur and professional, headbutting an opponent is no longer allowed. Muay Thai is unique in the way it uses all parts of the body, including the elbows and knees, for both training and competitions.

To strike and bind the opponent for both offensive and defensive purposes, small amounts of stand-up grappling are used: the clinch. Formal Muay Thai techniques are divided into two groups: Mae Mai or major techniques and Luk Mai or minor techniques. Muay Thai is often a fighting art of attrition, where opponents exchange blows with one another. This is certainly the case with traditional stylists in Thailand, but is a less popular form of fighting in the contemporary world fighting circuit. With the success of Muay Thai in mixed martial arts fighting, it has become the de facto martial art of choice for competitive stand-up fighters. As a result, it has evolved and incorporated much more powerful hand striking techniques used in western style boxing and the Thai style of exchanging blow for blow is no longer favorable. Note: when Muay Thai fighters compete against fighters of other styles (and if the rules permit it), they almost invariably emphasize elbow (sok) and knee (kao) techniques to gain a distinct advantage in fighting. Almost all techniques in Muay Thai use the entire body movement, rotating the hip with each kick, punch, and block. The rotation of the hips in Muay Thai techniques, and intensive focus on "core muscles" (such as abdominal muscles and surrounding muscles) is very distinctive and is what sets Muay Thai apart from other styles of martial arts.

From Wikipedia

To know Muaythai better

Muaythai is the devestating "science of 8 limbs" from Thailand developed over 1000's of years.
The exact date of the creation of this fighting art is not known but it is widely believed to be over 2000 years old. Many other martial arts are believed to be this ancient though many of them were only created in the last 100 years (Aikido, Taekwondo, Karate).
Muaythai used to be referred to as Pahuyuth (multi-faceted fighting style) a couple of hundred years ago, but muaythai itself is only a part of the whole Thai fighting system that covers weapons and groundwork which many refer to as Ling-Lom, though this is not the correct name for it. Many people do not know about this "no holds barred" of muaythai and there are very few people that can teach it.

The Thais are a tribal people and have had to defend their country from many invaders throughout their history. It has been developed year by year passing from teacher to student for as long as the Thais can remember. Muaythai is often referred to as the science of eight limbs as it employs not only punching and kicking techniques but elbows and knees as well.
It could be said that Muaythai currently is split into two parts, the ring sport and the martial art. The art side contains many techniques that cannot be performed in a ring with gloves on.

From muaythaionline.org