A very brief history of Audi 100 series
C1 (1969-1976)
The very first version of the Audi 100 was presented to press in late 1968. Completely engineered by Audi, without knowledge or allowance of VW it found the approval of VW in the end. Liked by customers, a two-door sedan (10/69) and a very beautiful Coupe (9/70) completed the lineup.
The C1 was also sold in the US, where it distinguished itself with remarkably poor build quality.
C2 (1976-1984)
The C2 Audi 100 was launched in 1976, with crisper styling and unusual inline-5 engine. Initially it was offered in a two-door sedan (a very rare sight nowadays) and a four-door sedan form. Late 1977 saw a launch of a first Avant that could be described as the first life-style wagon. The Audi 200 entered the market as the top model in 1978, the flagship of the family was characterised by luxurious features, sophisticated technology, and powerful engines. In the Audi 200 5T a 125 kW turbocharged five-cylinder ensured absolutely outstanding performance.
The C2 was sold as the Audi 5000 in the United States, in order to rebrand the car and avoid association with the C1. It was a sales success, allowing Audi to spread development costs over a much wider base than Europe-only competitors.
C3 (1983-1992)
Edging out the Ford Sierra as Car of the Year in Europe, the C3 had a remarkable aerodynamic look (achieving a drag co-efficient of 0,30) and marked a huge leap forward from the boxy shape of the C2. The two-door models were no longer available, and the Audi 100 Avant became more of a station wagon. The 200 continued as the upmarket variant. And for the first time the Audi 100 was now available with quattro permanent four-wheel drive.
At the end of the 1980s, the Audi V8 was announced. This was essentially a 200 Quattro with an engine derived from two four-cylinder Volkswagen Golf GTI engines put together. The V8 continued to be sold till 1994, when it was replaced by the A8.
C4 (1990-1997)
The successor of the C3 was not really a giant leap forward in late 1990. Interior and seats, as well as the engines and suspension were carried over at least partially from its predecessor. However, they did not look old, what shows how modern the C3 was. The name Audi 200 died on the European market. Top of the line was Audi S4, launched 1992.
From 1995 onwards, Audi dropped the 100 nameplate, calling it the A6 instead. In addition, what had previously been sold as the S4 became the S6.
Sources: audistory.de, wikipedia.org