From Knowledgebase

Quattro is Audi's trademark for its all-wheel-drive system.

Generations

All Audi quattros are full-time all-wheel drive except the Haldex-equipped models, which are usually the models with transversally mounted engines. Here is a general description of the evolution of the quattro system since its inception.

Gen	Description	Models
I	Open front differential Open center differential, manually lockable* Open rear differential, manually lockable*	1981-87 Audi Quattro turbo coupe 1984-87 Audi 4000 quattro 1986-87 Audi 5000 quattro VW Quantum Syncro
II	Open front differential Torsen center differential Open rear differential, manually lockable**	1988-91 Audi Quattro turbo coupe 1989-95 Audi 80/90 quattro, Coupe quattro, S2, RS2 1988-95 Audi 100/200 quattro, S4, S6
III	Open front differential Planetary gear center with electronically-controlled multi-plate locking clutch (Torsen center differential with manual-transmission) Torsen rear differential	1990-94 Audi V8
IV	Open front differential with EDL*** Torsen center differential Open rear differential with EDL***	1995.5 Audi S6 1996-curr Audi A4, S4, RS4, A6, S6, A8, S8 1998-curr VW Passat 4Motion
Haldex	Open front differential with EDL*** Haldex clutch center, electronically controlled Open rear differential with EDL*** Normally FWD, but Haldex direct torque to the rear when slip occurs	2000-curr Audi TT, A3, S3 2000-curr VW Golf/Jetta(Bora) 4Motion

*	ABS disabled when locked (if equipped)
**	ABS disabled when locked; automatically unlocks when speed exceeds 15mph
***	Electronic Differential Lock: uses ABS to selectively brake a slipping wheel

Content in part contributed by Ti Kan, from his personal website