The heavy bags used in boxing and mixed martial arts work on the same basic concepts, and in fact you can use either style to effectively train for both fight sports. However, the bags manufactured and distributed specifically as MMA bags do have some differences from traditional heavy boxing bags.
A standard MMA punching bag weighs 100 pounds when filled. Boxing heavy bags come in a variety of weights, ranging from 80 to 120 pounds. In most cases, a boxing bag gets its weight from cotton batting, sand and possibly metal weights at the center. A modern MMA punching bag uses some padding around a hollow core you fill with water. That water provides most of its weight.
The water core of an MMA bag gives the bag a different feel and texture from the solid padding that fills a boxing heavy bag. Mixed martial arts exploded into popular success after the invention of the water core bag, and practitioners embraced the new technology. The water core is easier on the hands and wrists of a fighter who uses it, which is one reason MMA fighters can train in the smaller MMA gloves and without hand wraps.
In general, a mixed martial arts bag will be longer and thinner than a boxing heavy bag of the same weight. This is because MMA competition permits kicks as well as punches. A regular boxing heavy bag allows practice for kicks to the head and ribs. However, the longer bag allows a fighter to practice the low thigh kicks that are a bread-and-butter move in MMA.
Free-standing heavy bags use a counterweight for stability rather than hanging from a ceiling. Boxers generally use this kind of bag only if they have to train in an area that doesn't allow a hanging bag to be installed. Mixed martial artists will use free-standing bags to practice a technique called "ground-and-pound." This technique involves kicking the bag until it overbalances and falls down, then mounting the bag to deliver a flurry of punches and elbows as if it were a prone opponent. Because of this, MMA fighters use both free-standing and hanging bags during practice.