A direct-shift gearbox is an electronically controlled, dual-clutch, multiple-shaft, automatic gearbox, in either a transaxle or traditional transmission layout, with automated clutch operation, and with fully-automatic or semi-manual gear selection. The first dual-clutch transmissions were derived from Porsche in-house development for the Porsche 962 in the 1980s.
DSG DL 501
Part-cutaway view of the Volkswagen Group 6-speed direct-shift gearbox. The concentric multi-plate clutches have been sectioned, along with the mechatronics module. This also shows the additional power take-off for distributing torque to the rear axle for four-wheel drive applications. (View this image with annotations)
A dual-clutch transmission (DCT) is a type of multi-speed vehicle transmission system, that uses two separate clutches for odd and even gear sets. The design is often similar to two separate manual transmissions with their respective clutches contained within one housing, and working as one unit. In car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.
DCT based on a concentric clutch layout (inside the round black cover, shown in the center of the image)