Warning! Broad generalisations follow!
In addition to what 090 said:
Pre-75 was designed for the bikes with 7" of front suspension travel, and 4" of rear. These were the bikes from the first boom time of MX, but had been made redundant by the longer travel bikes that started coming in in 1975. VMX began with this class (along with the older eras) because they marked the end of an era in the history of MX and the bikes were sitting around unloved.
Pre-70 was for the earlier era, basically before the Japanese made anything decent.
Pre-65 was for the era when the pommy (and occasionally Swedish) 4-strokes were the kings and there weren't really any decent 2-strokes.
Pre-78 is for the "medium travel" bikes that have more travel than the pre-75 bikes but are still disadvantaged compared to the later Evo bikes.
Evo is the category for the long travel bikes that don't have the 'modern' features like disc brakes, linkage rear suspension and water cooling.
Pre-85 is just that - these bikes are 'better' than even the Evo bikes, but still old enough to fit into the VMX world.
You'll notice that Evo is unique in being determined by technology rather than age, and it does create some messiness in the rules. But we all live with it because it works and is understood.
Bikes are generally classified on their newest major component. So if you had a 1984 frame with a 1969 motor in it, then it would be a pre-85 bike.
090 was probably right to avoid discussion about carry-over models, but I'll try to explain it simply...
Where a bike or a component that was manufactured after a particular cut-off date, but is identical (or very similar) to one made before the cut-off date, then its stupid to force the bike into a newer era. So those bikes/components are allowed as carry-over models, and are allowed to compete in the earlier class.
The other tricky bit is that some bikes are allowed as carry-over models even though the model didn't come out until after the cut off date. The Yamaha RT1 is an acceptable pre-70 bike, even though they weren't made until 1970. They're allowed in pre-70 because the technology is the same as the the pre-70 bikes and they're clearly inferior to the majority of the pre-75 bikes.
Hope that makes sense. The wording of the rules drives me insane, but the actual intent is extremely good and very close to the money (IMHO). The basic intent is to get bikes out of sheds and being used. The rules also try to get bikes racing against similar bikes to the ones they raced against when they were new.
While they're not perfect, the rules do quite a good job of acheiving these goals.
