Sit on Tops are one KIND of kayak.
They are the appropriate kind to use if you wish to try kayak diving.
They are also my PERSONAL preference for "all around" paddling and teaching.
The other kind of kayak (the ones most people think of) is known as a "cockpitted" kayak, and should be researched elsewhere if this is your preference. They are generally a faster-tracking boat for touring, but much less stable, more difficult to "master", and require specific training to safely use (such as launching the boat, entering the boat and what to do in capsizing situations).
I like the Ocean Kayak Sit-on-Tops, because they are VERY stable, easy to use, easy to teach, and great for having fun, which is what I like to do!
Here is an example of just how STABLE a sit-on-top Ocean Kayak is
(Try doing THIS on a "cockpitted" boat!):