Refractors are telescopes that use a lens in the front and an eyepiece in the back. Cassegrains are telescopes with a concave primary curved mirror in the back, a convex secondary mirror in the front, and then you look through an eyepiece in the back. Maksutov- and Schmidt-Cassegrains put corrector lenses on the front. Because of the complexity of these designs, they're more expensive than a Newtonian of a similar aperture, yet the Newtonian often performs just as good. Since these other designs have the eyepiece in the back, they require tripods to elevate them. Those tripods and their associated mounts are either much wobblier than a Dobsonian rockerbox, or much more expensive.