A schmidt cassegrain folds a long focal length into a short tube by bouncing light three times, and that compactness is why it dominates the tracking telescope market. It is the design most people mean when they picture a modern computerised telescope.
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How the light path folds
Light enters through a thin corrector plate, travels to a primary mirror at the back, reflects forward to a convex secondary mounted in the corrector, and returns through a hole in the primary to the eyepiece. Three passes down a short tube give a focal length two or three times the tube's length, so a 200mm f/10 instrument is about half a metre long instead of two metres.
What that buys and what it costs
It buys portability and a long focal ratio, which suits planets and makes ordinary eyepieces reach high magnification. It costs some contrast, because the secondary obstruction is larger than a newtonian's, and it costs cool down time, because the sealed tube takes a while to reach ambient temperature and the image is soft until it does.
Schmidt cassegrain vs maksutov
A maksutov uses a thick curved meniscus corrector instead of a thin plate, which is easier to make accurately at small sizes and heavier at large ones. Below about 150mm the maksutov usually gives sharper planetary images; above it the schmidt cassegrain wins on weight and price. Both are catadioptric, meaning they use mirrors and lenses together.
Questions people ask about telescope
What is a schmidt cassegrain telescope?
A catadioptric design that folds a long focal length into a short tube using a corrector plate and two mirrors. Common on computerised mounts.
Is a schmidt cassegrain or a maksutov better?
Maksutov below about 150mm for planetary sharpness; schmidt cassegrain above it for weight and cost. Both are compact and both need cool down time.
Is an SCT telescope good for deep sky?
Adequately, with a focal reducer to widen the field. Their long native focal ratio suits planets better, and a dobsonian of the same price gives more aperture for faint objects.