Loaded dice are polyhedra which land with one side facing upwards more often than one in the number of sides. A fixed loaded die is often made by putting a weight on the side opposite the desired face; usually buried just below the surface. In an amateurish die, you can see the circle of the cut used to remove the face and bury the weight. In a professional die, the weight is inserted in manufacture; in the case of a wooden die, this can be done by carving the die around a heavy inclusion, like a pebble which the tree has grown around.
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 likelyhood that the north side or the south side will land on the bottom depending on the direction of the current.
A heavily loaded die can seem almost willful as it bounces innocently along the table; then with an immense effort rights itself and hauls the six up to face an incredulous world.