Cyclists thinking about getting clipless pedals, especially starting out, may be overwhelmed by the plethora of choices that are available. This article will hopefully clear up the differences between certain kinds of clipless pedals, and also to explain the concept of "float".
Perhaps the most important thing to keep in mind is that clipless pedals can't really be discussed exclusive to themselves: they are part of a system that consists of the pedals, shoes and cleats.
Types of Clipless Pedals
Generally speaking, the two main types of clipless pedals are those made for mountain (MTB) bikes, and those made for road bikes. MTB pedals generally are double-sided (they have a binding on both sides), and have a more "open" design to allow clearing mud and other debris. Road pedals are typically one-sided and usually have a larger platform underneath your foot.
There are also both MTB and road shoes. MTB shoes typically have a lugged, stiff sole, with a recessed cleat, to make walking easier (much more so than road shoes). Road shoes usually have extremely stiff soles, with the cleat projecting from the bottom at the ball of the foot. Walking in them for any distance, or on a slick surface, is very uncomfortable and could be dangerous (you could slip!).
Although their names suggest otherwise, either kind of system can be used on any kind of bike -- after all, neither the bike nor the pedals knows the difference. Although there would probably not be much point in using road pedals/shoes on a mountain bike, many cyclists use MTB pedals and shoes on road bikes, because it makes walking (for commuting) much easier.
There are three things to consider regarding compatibility:
- Cleats and Pedals
- Cleats and Shoes
- Shoes and Pedals
Compatibility between cleats and pedals is relatively simple: just use the cleats made by the manufacturer of the pedals for the pedals that you have. Although there are third-party cleats available, they are usually not materially less expensive.
Compatibility between cleats and shoes is more of an issue. The issue comes down to whether the bolt holes in the shoe match the holes that the cleat requires. There are two main bolt patterns in use today: the Shimano SPD pattern (not the same as SPD-R) which uses two bolts, and the Look pattern which uses three bolts. (For example, when you see a shoe described as being SPD-compatible, what it means is that the shoe's sole is drilled with the appropriate  bolt holes for SPD cleats.) Almost all MTB pedals/cleats are SPD-compatible. Most road pedals/cleats are either SPD or Look compatible. (There is also a high-end road system made by Time, and a new high-end Shimano road system called SPD-R.)
Finally, there is compatibility between shoes and pedals. Sometimes, the design of a particular (lugged) sole on an MTB shoe may interfere with clipping in or releasing on a particular pedal, this is usually fixed with a Stanley knife, or something similar.
Early models of clipless pedals locked the rider's foot in a fixed position relative to the pedal (as if the foot were bolted directly to the pedal). While this produces maximum efficiency in power transfer, it can be very bad for the knees unless the cleat alignment is perfect (and even then, it may not work well for some riders).
Most current models of clipless pedals allow what is called "float". Float allows the foot to rotate a few degrees to either side of the center line without causing the binding to release. The exact amount of float depends on the pedal; some systems (i.e. Look) allow you to choose whether or not you want float, depending on the cleats you choose.
For most riders, pedals with float is the best choice, since it can help prevent knee injuries. You should note, however, that having pedals with float is not a substitute for having your cleats set up properly.
If you have clipless pedals, there a few things to be on the look out for (besides practicing clipping in and out):
- Make sure that bolts that hold the cleats to your shoes are tight. It is almost impossible to get loose cleats to release.
- Keep the pedal mechanism CLEAN and lubricated. It will not release properly if it is all gummed up and full of gunk.
- Check your cleats for wear periodically; this is especially important with systems (i.e. Look) that use plastic cleats.