What is the difference between a vocational tractor and an on highway tractor?

Vocational tends to refer to niche role vehicles - that could be anything from 6x6 off-highway prime movers to simple road tractors equipped with a boom and fork or grApple, to a straight truck, such as a dump truck, roll off truck, grApple truck, etc. These vehicles tend to be geared more for sitework, which may involve operations on unimproved surfaces, thus, they tend to be geared much lower. They're not intended so much for long distance operations.