
Bike tire tubes go in between the rim of the wheel and the tire which can be knobby for mountain biking, smooth for road bikes, or anywhere in between. The size of the tube is important and is typically recommended on the side of the tires. Tires can vary in size greatly so it is important to get the correct tube.
Road bike tire tubes are much thinner in diameter than a mountain bike tube but are designed to hold much higher pressures associated with road biking. Mountain bike tubes can be ridden at much lower pressures to ease the shock of bumps and for better traction, but pinch flats can become more common.
Bike tire tubes are slowly becoming obsolete as more manufacturers begin making their rims and tires tubeless. This allows them to run at much lower pressures without having to worry about pinch-flats because there is no tube between the rim and the obstacle that can be pinched. If a flat does occur it can require a completely new tire which may be troublesome for most people.