Clipless pedals are bike pedals for which there is a "click-in" system, an interface between a cleat on the bike shoe and the pedal itself. One of the most popular clipless bike pedals systems of all time is the Shimano SPD series, pictured below. You can see that the cleat is very small and triangular in shape, and it attaches to the bottom of custom made bike shoes. To engage, a biker pushes the tip of the bike shoe cleat into place, then snaps the remainder of the cleat into place. To disengage, a rider just needs to twist his foot, leading with the heel, away from the bike frame - and voila, easy out!
If you're like me, you probably find the term clipless confusing in this context. When I ride my Speedplay pedals or Look pedals, I distinctly feel like I'm clipping into the pedals! So why are all the modern pedal systems called clipless? It's just a carryover term from the pedal design which featured plastic or metal "cages" attached to the front of pedals, and a rider fastened his feet to the pedals with straps. These are called toe clip pedals. At least for me, I reckon the term clipping to be the action of attaching shoes to pedals, so I should say that I "click" into pedals, not clip into them. In any case, the term "clipless" - as referring to all the current pedal systems which involve a cleat that interfaces with the pedal - is still kind of confusing to me! Since, after all, one clips into the pedals. Here are examples of toe clip style pedals:
So if you want to describe the opposite of clipless pedals, it would be clip pedals, but more accurately you would say, pedals with toe clips. (And straps.)
Some of the most popular modern pedal designs, all of which are clipless, are:
For more information on clipless pedals, see: