There are many different types of pedals available these days for road and mountain bikes. Most adult bikes require a pedal with a 9/16" thread and are fitted using either a 15mm spanner or a 6 or 8mm Allen key. Some of the most popular types are listed below.
Flat pedals – these can be made of plastic or metal. Metal pedals tend to offer more durability and grip. Some pedals come with sealed bearings on the axle making them smoother, and this also helps prevent water ingress and damage. Price points range from 5 - 300.
Wider metal flat pedals are primarily designed for BMX and Downhill/Freeride cycling and come with pins which are designed to offer more grip to stop your foot moving on rough terrain and in wet weather.
You can add toe clips and straps or half toe clips to some flat pedals and this can improve power when cycling if your foot is held in position. This type of system is popular on town and city bikes where you don't need to remove your feet often.
Clipless or SPD (Shimano Pedalling Dynamics) pedals – are designed to operate with cleats. These are fitted to the bottom of the sole of a SPD shoe and you click the two together whilst pedalling. SPD systems are popular with recreational cyclist's, commuters and mountain bikers as the cleats are metal making them durable and also the pedals are easy to clean mud and dirt out of. An SPD type pedal will keep you attached to the pedal with a ski binding type system, which you flick out your heel to release your foot.
Using SPD pedals and shoes will make your pedal stroke more efficient and add power to your cycling. Some schools of thought suggest that by using the best SPD type pedals and stiffest SPD shoes you can improve your efficiency by as much as 7% over a pair of trainers. So if you rode for 100 miles you would theoretically have got 7 miles for free.
Most SPD pedals have a cleat fitting on each side. Pedals are also available though with one side for cleats and one side flat. These are popular with commuters as it means you can ride your bike either clipped in or with normal shoes/trainers.
Road bike clipless pedals come in various different designs. The most popular "standards" are Look Keo, SPD'R, SPD SL and Speedplay – these pedals are designed specifically for road use and are lightweight and aerodynamic in construction. There are subtle differences between all of the systems and some people prefer one type over the other however they are all well-known and respected within the bike industry.
The main differentiating preference between people who buy road pedals is "Float" which means the degree to which your foot moves around when clipped into the pedal. If using a Look pedal system, the float can be adjusted by changing the actual plastic cleat, Black cleats offer zero-degree float, Grey cleats offer 4.5 degrees' float, Red cleats offer 9 degrees' float. The amount of float is down to personal choice. If you have bad knees then it is advisable to go for pedals with more float so you can find a comfortable position more easily.