Lovely chunky beasts. The European ones are a different species to the Americans, but can interbreed. They are very similar, but have some differences.
1) On average taller than American bison, and have longer legs.
2) Browse more, and graze less, as their necks are set differently.
3) Nose set further forward than the forehead when the neck is in a neutral position.
4) Body less hairy, though its tail is hairier.
5) Horns point more forward, making them better at fighting with interlocking horns, like domestic cattle, unlike the American bison, which favours charging.
6) Less tameable than their American cousins, and breed with domestic cattle less readily.