I have written a tutorial on the differences between the types of Kilts that are currently out there.

http://xmarksthescot.com/article/sa1.php

Basically a Traditional Style Kilt is Hand Sewn, will fit with the waist fastened just under the ribs and from Good Quality Worsted Wool in a Registered Tartan.
The Hallmark of a Traditional Kilt is the internal interfacings and strengheners built into the Kilt and hidden by the liner that give the Kilt its ability to not stress the stitching and insure a long life. The nickname of a Full, hand-sewn, 8 yard Traditional Kilt is a "Tank" because they will wear like one and last a lifetime.
All Traditional Style Kilts should be custom made.

A Casual Style Kilt will look very similar to a Traditional but may be Machine Sewn, may be made from a fabric other than Wool or Tartan, and may use fewer, wider or shallower pleats. Some casuals can be worn lower, at what I call mid-rise.
The hallmark of a Casual is the lack of interfacings and strenghenings. Most Casual Style Kilts rely on the strength of the stitching to hold their shape.
A Casual Kilt can be easily taken for a Traditional at first glance.
Casual Style Kilts can be custom made or off-the-rack.

A Contemporary Style Kilt is one where the interfacings and strengthenings are incorporated into the Kilt but in a different manner than a Traditional. Usually inside the waistband. Contemporary Style Kilts can be made from Tartan Wool but usually are made from a solid color fabric of a Polyester/Cotton Blend or 100% Cotton similar to "Dockers" slacks.
Contemporary Kilt usually have Pockets incorporated into their design, eliminating the need for a Sporran. A Sporran may be worn with a Contemporary Kilt if the wearer wishes and most people will not be able to tell on first glance that it is anything but a Kilt.
A Contemporary Style Kilt is usually custom made.

A MUG is an Alternative Garment that resembles a Kilt. They may have pleats and aprons but are designed to be worn differently than a Kilt and usally in a style other than what is the conventional Kilt manner. MUGS usually do not have any interfacings or strengthenings built into them. All the stress of wearing them is taken up in the stitching. Similar to Jeans construction.
The hallmark of a MUG is how it is sold and intended to be worn by its designer. Goth wear, Grunge wear and the like are some examples.
A Mug is almost always sold as off-the-peg by waist size.