Through the 1964 model year, VW used consecutive chassis numbers. However, in 1965, VW went to a standardized 9-digit chassis number and the model year and style was identified with the first 3 digits of the chassis number.
The first number, 1, identified the model as a Sedan chassis (thus Beetles and Ghias would both have 1s in the first position). The second position identified the style:
1 - Standard / Deluxe Beetle (except during 1971-1972 when Super Beetles also had a '1')
3 - Super Beetle during 1973-1979
4 - Karmann Ghia
5 - Convertible
The third number identified the year from 1965-1969. From 1970-1979, to not duplicate numbers, VW added a 2 behind the year and in doing so went from a 9-digit to a 10-digit chassis number. This stayed the same through the end of production.

Commonly asked questions:
Q - My Beetle has one number on the transmission tunnel under the rear seat and one on the body and they don't match.
A - Two possible answers. If the rear window is a split or oval style, then the car is a 1946-1955 model. During this time, VW used two separate numbering systems for the body and the chassis and, though they were sequential, they did not match as this system went back to the war when different bodies were put on Beetle style chassis. The other possibility is the body and chassis was changed at one time. This happens a lot due to a body being swapped for various reasons (rust, accident, etc) or the chassis was upgraded by the previous owner for suspension improvements (for example, an IRS pan under a 1964 Beetle).

Q - I have a 1980 Beetle Convertible but the chart lists it as a 1979 model. Whose correct?
A - Technically we both are. Some 1979 models were brought in and tagged as 1980 models for various reasons. However, for ordering parts, your car is a 1979.

Q - My Beetle was made in Dec of 1973, why is it a 1974 model?
A - VW ran it's model year from Jan-Dec until 1955 when it changed to a August-July model year. So, a Beetle made in December of 1954 is still a 1954 model, but a December of 1955 is a 1956 model (and why your Dec 73 is really a 1974). However, this was only the case, as far as we know, in the US. In Germany it was kept Jan-Dec which is why you'll have a European 66 model with upright headlights, etc., like a 1967 US model.