A plug is a real race boot, what the sponsored guys have, or at least very close. It is characterized by very thick plastic, especially in the lower, that is intended to be nothing like a human foot, but leave much room for grinding and punching to ideally result in a spray-on fit. Consumer boots have much thinner plastic.
Also, the boots tend to be uncomplicated in design, usually with a one-piece lower and a one-piece upper, secured with a rivet instead of the goofy cuff cant adjusters on consumer boots. A proper cant is only achieved by sole planing anyway... Plugs tend to be a little more upright, on average, than consumer boots, and tend to be stiffer. Also, due to the thick plastic and very thin (often leather) liners, they are very cold. They are the best-performing ski boot you can buy.
Examples: Salomon Course X2, Nordica Dobermann WC 100 and 150 (NOT the pro 110 and 130), Atomic RT Ti (Not the CS), Head RD96, Tecnica Diablo Race R, etc.
A semi-plug is basically a watered-down plug. Still somewhat above a consumer race boot like a salomon course spaceframe, but not a full plug. They are a little wider, have a little more padding (most plugs have none except at the ankles and tongue), and are softer. They offer most of the performance without the 7 (for me) hours of grinding and stretching to get into the boot.
Examples: Tecnica Race Pro 110,130. Atomic RT CS, Nordica Dobermann 130/110
A consumer boot has a thicker liner, a thinner shell less tolerant of grinding, and is significantly softer, on average. The shell is usually made up of multiple pieces (3+), sometimes with elastomer layers for shock absorption. The contrast is illustrated here:
RT CS (semi plug)
Note the 2 -pice shell and lack of shock-absorber garbage.
M10 (consumer "expert" boot)
You can see the contrast, how the shell is made up of many pieces and that rubbery stuff is everywhere.
Hope that helps.