Off-road tyres generally have bigger tread blocks with large grooves between them, enabling the tyre to bite into loose surfaces for better traction. The downside of this is that they perform less well on-road. You should expect fuel consumption to increase, less grip on normal roads and a significant increase in noise levels, In terms of wear, the more they are used on-road compared to on-road, the more quickly they will wear out, and you should expect them to wear more quickly than an equivalent road tyre.