Mountain bikes often have suspension and knobbly tyres and are designed for off-road trail biking. They usually have a lot of gears so that the bike can be ridden in difficult terrain. They generally have flat handle bars and require you to hunch over, which works well for difficult off-road tracks.
These types of bikes are also seen in urban areas, often with modifications to make them more suited to the on-road environment. With these modifications, they are sometimes called ‘Hybrid’ bikes. They do not come standard with items like mud guards and back racks so you will need to spend extra to get these things fitted if you intend to use the bike for everyday riding as well as off-road riding.
This type of bike is currently the ‘default’ type of bike in New Zealand. Make sure that this is ‘actually’ the type of bike you want when you go shopping, rather than the type the shop assistant wants to sell you just because it’s what they have in stock … or on special.