The Urban mountain bike is a bike that is designed for riding in the city. They differ from a regular mountain bike several ways.
They usually have rigid forks for a responsive lightweight and lo-maintenance ride.
They have taller gearing better suited to riding on the road, most will have a 48T front chainring rather than the usual 42 or 44T on a regular MTB.
They come with slick tyres for fast efficient commuting.