At one time, Modern Maid, GE, Roper, Amana and Jenn-Air all produced a downdraft range. Now Jenn-Air is the only manufacturer left manufacturing this product.
A downdraft range features an integral blower inside the range itself. The smoke, grease and heat is then drafted down through the range to the outside via ductwork. Quite simply, it is exactly reverse of a regular range hood, except the air is being pulled down rather than up.
The main advantage to a downdraft is no additional vents are required. If the stove is in the middle of an island or peninsula or you can't vent through the ceiling, then the downdraft is your only viable option.
Downdrafts do have their disadvantages. The blower is only 310 CFM or roughly equivalent to an over the range microwave. Because it is pulling smoke, this may be undersized for high-volume cooking. Downdrafts also do not have the capture area of a regular vent as well so they're not as efficient.
For these reasons downdraft ranges should be considered a last resort.
Jenn-Air Downdraft Ranges:
Jenn-Air has actually updated its range over the last few years. On the gas side they now feature a 16,000 BTU burner as well as a 500 BTU simmer. On the electric side, they added an all Ceran top.
The basic difference between the Jenn-Air ranges are whether you want to grill option with convertible gas or electric burners or a regular range with four permanent burners. Jenn-Air also has a dual fuel option as well as a basic range without convection.
Download the Yale Ventilation Buyers Guide for more detailed information on downdrafts, ducting and ventilation.