A tall tub dishwasher is one that has been modified to have a larger interior capacity while maintaining the outside dimensions.

Most dishwasher manufacturers offer both the standard as well as tall tub models. The tall tub models are designed to have a larger interior capacity. Most standard models are designed to hold up to twelve place settings while a tall tub can generally hold up to sixteen. This is an increase in dishes holding capacity of more than 33%.

Benefits/Disadvantages of a Tall Tub Dishwasher

The obvious benefit to purchasing a tall tub model instead of a standard is that you will be purchasing a unit that has the same dimensions on the exterior, but has been modified on the interior have more dish holding space.

The disadvantage to having a tall tub dishwasher is that it may not be optimal for small families or single individuals as the larger interior will take longer to fill, which may force it to be run when it is only partially full. 

Example Dimensions

Maytag Tall Tub Capacity
(Model: Whirlpool Gold® Series Dishwasher with Sensor Cycle | WDT910SSY)

Depth 24 1/2" (62.23 cm)
Min. Height 33 1/2" (85.09 cm)
Max. Height 34 1/2" (87.63 cm)
Width 23 7/8" (60.64 cm)
Weight 91 lbs (41.2685 kg)

http://www.whirlpoolappliances.ca/en/ProductDimensions/WDT910SSYM.html 

Maytag Standard Capacity
(Model: Whirlpool® Built-In Dishwasher: Black-on-Stainless | DU850SWPS)

Dimension inches (cm) / lbs (kg)
Depth 24" (60.96 cm)
Width 23 7/8" (60.64 cm)
Min. Height 33 7/8" (86.04 cm)
Max. Height 34 7/8" (88.58 cm)
Maximum Undercounter Height 34" (86.36 cm)

http://www.whirlpoolappliances.ca/en/ProductDimensions/DU850SWPS.html