bike pedals..

A bike pedal is the part of a bike that the rider pushes with his foot to propel the bike. It provides the connection between the biker's foot or shoe and the crank allowing the leg to turn the bottom bracket axle. Pedals usually consist of a spindle that threads into the end of the crank and a body, on which the foot rests or is attached, that is free to rotate on bearings with respect to the spindle.

Flat and Platform Pedals 

Platform pedals are pedals with a relatively large flat area for the foot to rest on, in contrast to the quill pedal which has very little surface area. In mountain biking and BMX, platform pedals typically refer to any flat pedal without a cage. MTB and BMX riders prefer platforms to cage pedals because they offer more grip using short metal studs, are rounder and do less damage to a rider's shins and ankles during an accident. Cage pedals often scrape skin and flesh off the shin if the rider stops short and loses footing of the pedal.

Quill Pedals

Cages refer to pedals that can accommodate toe clips and not necessarily the clips themselves, and are more specifically called quill pedals. The actual cage is the outer part of a conventional pedal, the part that comes in contact with the rider's shoe and has holes where toe clips can be secured by screws. Toe straps used with clips further secure the foot to the pedal and to allow the cyclist to apply power on the upstroke by pulling against the pedal. The main difference between track, road, and touring quill pedals is width. Track pedals are narrow and the front and back plates of the cage are separate, road being a little wider with a one piece cage in a shape of a sideways "U", and touring being the widest to allow for comfort when used with wider, non-racing shoes during longer rides.

Clipless Pedals

Clipless pedals (also clip-in or step-in) require a special cycling shoe with a cleat fitted to the sole, which locks into a mechanism in the pedal, holding the shoe firmly to the pedal. Most of today's clipless pedals lock to the cleats when stepped together firmly, and unlock with when the foot is twisted outward. Clipless refers to the lack of an external toe clip (cage), but not to be confused with platform pedals without toe clips.

- Wikipedia