Ive ridden Look Keos, Shimano SPD-SL, and my current ride, Speedplay zero.

All pretty much function the same, they hold your feet to the pedal, simple as that. I preferred the Keos over the SPD-SL mainly on how the float felt.

I love the zeros though. Double sided and easy to adjust. Fore/aft and left/right cleat adjustment is independent, so makes setting up your cleats relatively easy.