The fork is the part of the bike that holds the front wheel and connects to the handlebars to control steering.
Rigid forks are most common on road, urban, cruiser, and hybrid style bikes. They can be made of steel, titanium, aluminum or carbon fiber. Rigid forks have good power transfer and make the bike feel stiffer and more responsive overall.
Suspension forks are found most commonly on mountain bikes and some hybrid bikes. They use coil springs or air pistons to modulate a certain amount of travel; i.e. the upper stanchions of the fork can travel into the hollow lowers of the fork.