The first generation, or Mark 1 (Mk1), Golf was released in 1974 and was to replace the aged VW Beetle. The Golf (the name had nothing to do with the sport but is the German spelling of Gulf, as in Gulf Stream) differed very much to the Beetle mechanically as well as aesthetically, moving from a rear-mounted air-cooled engine to a front mounted liquid-cooled engine using technology that came with purchasing Audi (that's why many Mk1 engine components have Audi logos stamped on them). The end goal, however, was the same: to build a small, economical, and affordable family car.
Several variations of the Golf were made, as follows.
The Golf was never meant to be fast, let alone a sports car. As a kind of gimmick, engineers at the Volkswagen factory fitted a light, little Golf with a Bosch mechanical fuel injection unit, usually reserved for sports and upmarket cars. The project caught the eye of a few higher-ups and a short production run was arranged. At first, VW execs were cautious of the idea and only intended to sell the quick GTi in Germany but demand from other countries, particularly the UK, influenced VW enough to take production seriously. Thus the concept of the 'hot hatch' was born.
GTis feature either a 1.6 or 1.8lt mechanical injected SOHC engine, slightly lowered suspension, slotted front discs, anti-roll bars, and are distinguished by a red trim line around the grill, tartan patterned seats, and commonly additional gauges in the centre console. GTi stands for 'Grand Touring Injected'.
Special edition GTis
A quick note about Bosch fuel injection systems.
Just about all fuel injected Mk1 Golfs use a version of the Bosch K-Jetronic system. This is a mechanical fuel injection system which was very popular on European cars from the 1970's through to the 1990's. It is known by a few names: Jetronic, K-Jet, or CIS (Continuous Injection System). Digifant may also be used, but this refers to an early computer management system.
The Cabriolet is simply the convertible version of the Golf. Sometimes these weren't badged as Golfs at all - just VW Cabriolet. These cars were not actually made by VW, but were built by the independent car factory Karmann (known, among other things, for the Karmann Ghia, which actually uses many components from the Beetle).
They were popular cars and, as a result, the Karmann factory kept churning out Mk1 Cabriolets well passed when Volkswagen themselves ceased production of the hatchback. That's why it's possible to buy car made in 1993 closely resembling the same model first made in 1974. Later model Cabriolets were fitted with body kits, the 1.8lt injected engine, and different, newer interiors in an effort to give the aging car a more modern appearance.
All Cabriolets came from the Karmann factory, and all Cabriolets built during the Mk2 Golf hatchback run are still Mk1s, though they are colloquially called Mk1.5s.

Photo by Conor Samuel on Unsplash
Due to it's long production run (1979 - 1993) there were many, many different special edition Cabriolets. You can read about special edition Cabrilets here or on Cabby Info's excellent site.
"Mk1.5" is a colloquial term for the Karmann factory built Cabriolet. When its production run coincided with the Mk2 Golf hatchback Karmann added some Mk2 components, like the alloy wheels.
Refers to the generation of Mk1 Cabriolets that received the plastic colour-matched "Clipper" body kit introduced in 1988, replacing the chrome bumpers. The cars often also utilised the larger 1.8lt injected engine.
Diesel Mk1s were first manufactured in 1976. Performance wise these cars were very comparable to smaller petrol engine Mk1s. Many engine components are also shared between both diesel and petrol models.
The GTD was the sporty version of the diesel Golf and incorporated the GTi's suspension. Extra power came from a Garret turbo, providing very competitive performance.
The Volkswagen Rabbit was name given to the Golf in the USA and Canada, built in Pennsylvania. They are essentially the same car as their European counterparts, though some later models had square headlights extra side mounted indicators. The Rabbit name was later dropped.
Special edition Rabbits
This was the name given to Mexican produced Mk1 Golfs. Later models shared the same square headlights as the North American Mk1.
Special edition Caribes
Refers to early versions of the hatchback that have a slightly different design in the rear hatch between the taillights.
Citi Golfs were made in South Africa between 1984 and 2009. They are still, fundamentally, Mk1 Golfs but with some subtle changes, including a slightly sloping front grill, different C pillar design, and integrated rear window wiper. The most significant change was the addition of a new Skoda dashboard in 2004.
Due to its long production run, a huge number of special editions and special trim level model Citi Golfs were produced.
Citi Golf trim levels and special editions
Caddys are not technically Golfs. They're the pickup or ute version of the Mk1 hatchback (golf caddy - get it?). They are still considered Mk1s though and share many, many parts being almost incidental except for the rear body work. The Caddy name still lives on in the modern Golf-based small van.

Photo by Tim Meyer on Unsplash
Before the Golf GTi there was the sporty Scirocco coupe. In fact before the Golf, there was the Scirocco. VW used the sportscar to sort out any issues with the shared drive train before the Golf went into full scale mass-production.
The Scirocco is not really a Golf either, but they did share the engine, drivetrain and many other components. The body panels are the only major difference.
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