h2ohhh is correct. A circle hooks works by rotating and embedding itself in the corner of a fish's mouth as the fish turns and swims away after taking the bait. It is an entirely different process than having to "set" (aka jerk, pull, rear back) on a conventional "j" hook.
With conventional "j" style hooks - you have to "time" the hookset perfectly, and know when to strike and when to wait. With circle hooks, you just let the fish run with the bait and tighten up the line on him, and he hooks himself. Hard to learn to do, but very effective and actually much easier when you get the hang of it.
Commercial long-liners have used circle hooks for years for this reason. Just in the past few years have they become available to recreational bait-fisherman. And that is my final point - you ONLY use circle hooks with live or dead natural bait - never on artificial lures, since the fish don't hold on to them long enough to make them work.