A Twin-Lens Camera is a medium format camera that uses 120 or 220 roll film. An accessory item for some models would allow the use of 35mm film as well. The cameras are very quiet in operation due to no mirror movement and the fact that they use internal shutters. On cameras that have flash connectors, (x sync), the flash will synchronize at all shutter speeds. The primary method of viewing and focus is done through a waist-level finder. This finder show the image and most of the time there will be a split-image or other focusing aid. There is a magnifier in the finder for even better focus control.
Cameras such as Rolleiflex, have optional Prism Finders that could be purchased separately. These finders are very helpful for aiming at moving objects. Using the waist-level, the image is reversed. Using the prism puts the image back to a normal view.
Most camera used a crank to advance the film and cock the shutter. Early models may have knob advance and separate cocking of the shutter.
Lenses generally are normal range lenses. There were optional wide and telephoto adapters that attached to the lenses. Rollei had a camera that only had telephoto and another that was wide-angle.
Mamiya had several lines of interchangeable lens, twin-lens cameras. There were wide-angle and telephoto lenses available. These also had eye-level finders and metered finders available.
Rolleiflex, Yashica and others had twin-lens cameras that used 127 film. This was a good size for taking transparencies that could be projected in a 35mm slide projector.