The Yashica-24 is a Twin-Lens Reflex (TLR) camera designed to accept 220-size rolls of film. It will also accept 120-size film. The negatives or slides it produces are 2-1/4 inches or approximately 6cm square. This format is commonly called "6x6". A 220 roll of film can hold up to 24 such exposures, hence the "24" in the model name. A 120 roll, naturally, holds 12 exposures of 6x6 negatives or slides.
In the photos below, the directions "right" and "left" refer to the photographer's view from behind the camera.
The right-hand side of the camera holds the winding crank and the frame indicator (the small circle above and forward of the crank, just below the strap bracket). The crank unfolds and is cranked forward (clockwise) to advance the film (the mechanism locks when the next frame is reached) and then backwards (counter-clockwise) to cock the shutter. This returns the crank to its starting orientation so it may be folded back into storage position. This is true regardless of how far the film had to be wound to advance to the next frame. The thumbwheel just visible at the near end of the nameplate at the upper front of the camera calibrates the light meter to the speed of film being used.
The left-hand side of the camera looks a bit more complicated, but there are really only two controls used while film is loaded. The large knob is the focus adjustment and below it, the small button with a red end is a momentary type switch to operate the light meter. The two smaller knobs at the upper rear and lower front of the left side of the camera hold the takeup and supply film spools, respectively, in position. They pull out against spring pressure and are rotated to either keep them retracted while unloading, or to snap back down and engage the spool center when reloading. The slotted disk is the cover for the light meter battery compartment. At the end of the nameplate on the upper front of the camera is the photocell for the light meter.
Here is a close-up of the twin-lens assembly. The upper lens is the "viewing" lens and the lower lens is the "taking" lens. The right thumbwheel (left in picture) sets the shutter speed, from 1/500s to 1s plus "B". The left thumbwheel sets aperture, from f/3.5 to f/32. The small lever with the red dot engages a 10-second delay timer. The small lever with the yellow dot adjusts flash synchronization between "M" and "X". The shutter release is on the lower right and the PC jack to connect a flash unit is on the upper left.
220 and 120 are the same width (about 63mm), but 220 is about twice as long as 120. Also, 120 film has frame-indexed backing paper behind the film for its entire length while 220 only has backing paper at the head and tail ends of the film. Thus 120 and 220 use the same size spool, but a camera that accepts 220 film must be completely light-proof and provide its own means for indicating the frame-index of the film. Otherwise, the "ruby window" of a camera designed for 120 would expose the unprotected film in a 220 roll.