Canoes are typically built on an external strongback - essentially a rigid beam on which the forms are mounted above the strongback. Kayaks may use an external or internal strongback. An internal strongback is a beam where the forms for the boat are placed around the beam such that the beam pierces the form and the boat is built around the beam and form structure. A few years ago I purchased an external strongback along with the forms for a Kipawa canoe. I have never quite gotten around to building the canoe but since I had the strongback I thought that I might as well put it into service as a very rigid platform to support the internal strongback for my kayak, much as Dale did (in fact it was seeing his web site that gave me the idea and convinced me to ditch the sawhorses). An internal strongback is usually supported on a pair of sawhorses.
I find it interesting that there is such a difference in opinion out there. The canoe-building folks and followers of Ted Moores in particular, seem to believe that the strongback has to be incredibly rigid and even epoxy the strongback to the floor to prevent it from ever shifting, even though a strongback built to the Moore's design is an incredibly solid piece of wood. The kayak-building folks, and followers of Nick Schade in particular, opt instead for using a crooked 2x4 supported on a couple of rickety sawhorses as their backbone for building. I guess I chose to combine the methods. Rather than use a 16' 2x4 (ever see a straight one?), I chose to make a 2"x4" box beam strongback out of 3/4" plywood. The resulting beam is not as rigid as I might have expected, but at least when it's done wobbling it returns to a straight line. Although I had made a pair of sawhorses, I am instead mounting the internal strongback on supports mounted on the Moores'-style external strongback. Since this provides me a nice rigid platform to build on, I have mounted the whole apparatus on 4 casters so that I can move it around in my space-limited shop. Sure, it might flex a couple of thousandths of an inch as I move it around, but it should still be better than the sawhorses.
For an interesting and evolving perspective on a boxbeam-type internal strongback, see Bryan Hansel's comments during the building of a kayak of his own design. He started off hating the internal strongback, but by the end he seemed to like it.