Look: one of the more common road pedals, 3 bolt pattern for road shoes only; There are 3 different colored cleats, each color designating the amount of float. Black = 1 degree, grey = 4.5 degrees, red = 9 degrees. Cleats about $16, the ones with the cleat covers included about $20, pedals about $80 up
SPD-SL: Shimano version of Look pedal, 3 bolt pattern for road shoe only. Red/black cleat = about 3 degrees of float, Yellow/black cleat = about 9-11 degrees of float
The pedal is single sided and is weighted to ride nose high so you can nose the cleat in easier. Cleats about $24, pedals about $80 up.
Both of the above use larger cleats which may make getting clipped in easier. They are also single sided pedals and are weighted to ride nose high so clipping in is easier.
Speedplay: Has the X-series and the Zero series. For the X-series there is 28 degrees of float with no adjustment. Of the two, it is recommended for women and smaller riders as it is easier to clip out of. The Zeros series has adjustable float from 0 to about 20 degrees. The cleats for the two are NOT interchangable. Both offer a small pedal & improved cornering as well as a large cleat/pedal surface area to minimize hot spots. Great for those with knee problems (esp the X-series). Cleats about $45, pedals roughly $120. Your cleats contain the binding mechanism not the pedals which is why they are more.
SPD; 2 bolt pattern, can be used with mountain or road shoes. most pedals are double sided with the exception of one (single sided road version). Need to use SH-71 cleats on road shoes as it has a pontoon on either side of the cleat to stabilize foot when walking. cleats: $20-25, SH-71 cleat about $30-35, pedals about $55 up (float is roughly 6 degrees)
Crank Brothers: 2 bolt pattern, mountain or road shoe. Like the SPD, there is a version with the pontoons for road shoes. Float is somewhere between 6-13 degrees, cleats about $20-25, pedals about $70 up
The more float a cleat has the less chance of knee problems because the foot can move thru its normal motion. As far as shoe go: (1) mountain - uses 2 bolt pattern cleats which are recessed into the sole for easier walking; sole stiff but has some flex, Any pedal system not using the 2 bolt pattern cannot be used. (2) road shoe - will handle most pedal systems, very stiff & has better power transfer to pedals than mountain shoe. You balance on heel and the cleat which is where the pontoons come in handy for the eggbeaters & SPD cleats. The Speedplay, Look, and Shimano SPD-SL pedals all have cleat covers to protect the cleat and make walking easier.