Some pedals (usually MTB) also have 'lateral float' which allows the foot to move from the inside to the outside of the pedal. - TFcmatcan said:float refers to the number of degrees that you can rotate your toe, and therefore the angle of your knee, when clipped in. some pedals have none, some pedals have lots. lots of float options is important especially if you have a history of knee-problems or any conditions like uneven leg length. look pedals typically have good adjustable float (with adjustable float, you can lock in a setting for the amount of lateral movement you want from your pedals.)
so pro: less knee stiffness, good for people with knee problems/older riders
con: you have to work a little harder at your spinning technique to be as efficient, because your legs can wobble around too much. but if you get used to it and you have a good spin, it's a non-issue. i always recommend clipless pedals with some float options.
hope that helps. happy riding!