A variable loaded die is hollow with a small weight and a semi solid substance inside, usually wax, whose melting point is just lower than the temperature of the human body. This allows the cheater to change the loading of the die by breathing on it or holding it firmly in hand, causing the wax to melt and the weight to drift down, making the chosen opposite face more likely to land up. A less common type of variable die can be made by inserting a magnet into the die and embeding a coil of wire in the game table; then either leave the current off and let the die roll unchanged or run current through the coil to increase the likelihood that the north side or the south side will land on the bottom depending on the direction of the current.
One has to be very skillful with one's "moves" as a crooked die must be surreptitiously switched for one already in play. Needless to say, persons caught cheating at dice risk bodily injury or worse.
Transparent acrylic dice, used in 100% of reputable casinos, are harder to tamper with and smart players in private or military games insist on them.
It is unlikely that you would encounter loaded or other crooked dice at a licensed casino in the U.S., since they make plenty of money on the percentages and they would be foolish to risk revocation of their licenses.
See also dice.