These reflecting telescopes unify advantages which are otherwise only found at refractors (full aperture free) with the comparatively low prizes of reflectors and the advantages of that scopes (no spectra, etc). This is accomplished by moving the secondary mirror slightly out of the optical line of the primary one, thus avoiding any obstruction of the free aperture.
Actually, there are several variants of Schiefspieglers. They have in common that, contrary to the "conventional" all-coaxial (or rectangular) optical architecture (such as refractors [even binos with their prisms], reflectors of type Newton, Cassegrain and similar, or Schmidt, SC, Maksutow and other catadioptics), they reflect the light in "dubious" but well-calibrated angles. For this reason, their appearance is quite unusual, usually :).
Some Schiefspiegler variants are
Deviating from this nomenclature, which is common e.g. in Europe, at least some Americans speak of Schiefspieglers only if talking about Kutter type telescopes.