Large format cameras use film that is 4" x 5" or larger. The most common sizes are 4" x 5", 5" x 7", 8" x 10". Even larger cameras sizes do exists, but they tend to be more for special purposes. The most common size is the 4" x 5".
Large format cameras have several advantages over other types of cameras. First, the film size is much larger than the other formats. This means that less magnification is necessary to produce a print, resulting in much sharper and/or larger prints. Second, the movements of the front and rear of the camera allow for changes in perspective, plain of focus and positioning of the image. One example of perspective control is the ability to take a picture of a tall building without the sides of the building converging at the top. An example of manipulating the plain of focus is a shot taken close to the ground with everything in focus -- from a foot or so from the camera, all the way out to infinity. The third major advantage of large format cameras is that each shot is taken on an individual sheet of film. This provides the photographer with the option to manipulate processing on an individual shot basis, thereby giving the photographer more control over the contrast and tonality of the final negative or transparency.
One major disadvantage of the large format camera is the bulk. Large format cameras tend to be heavier and bulkier (harder to pack) than any of the smaller formats. A second disadvantage of large format cameras is that they are much slower to operate. For example, focusing takes longer and is usually accomplished by viewing the upside down and backwards image on the ground glass (i.e. the focusing screen) through a magnifying loupe while covering your head with a dark cloth to block out extraneous light.
But, if you want large, tack-sharp prints with the best range of tonality and detail, then large format is the way to go. My preferred large format camera is the Arca Swiss F-Metric 4" x 5". I use a wide variety of large format lenses from Rodenstock, Nikon, Fujinon and Schneider.
Large format digital photography is still in it's infancy. Of course, one can attach a medium format digital back to a large format camera when view camera movements are needed. But the truly large format digital sensors today are scanning sensors. This means that they slowly scan across the 4" x 5" image area. This requires the subject to remain absolutely motionless for a period of several seconds. Therefore, these cameras are useful for product photography, architectural photography and art copy/reproduction. But they are limited in their ability to capture outdoor scenes where the subject is rarely completely motionless.
Medium Format (Film and Digital Back)
Medium format cameras use film or sensors that are larger than 35mm but smaller than 4" x 5". There are a variety of aspect ratios found in medium format cameras. The most common film sizes are 6 cm x 6cm, 6 cm x 4.5 cm, and 6 cm x 7 cm. The most common digital format is approximately the size of 6 cm x 4.5 cm.
The main advantage of a medium format camera is that it uses larger film or digital sensors than 35mm-size camera. The larger film or sensor size (about 3x 35mm) allows larger prints to be made. Medium format cameras also retain some of the ease of use of 35mm camera systems.
The main disadvantage of medium format is less depth of field than 35mm/DSLR cameras without the movements of a large format camera. But, when movements are necessary, the medium format film or digital back can be attached to a large format camera. This is particularly helpful in macro work and commercial product and architectural photography.
My preferred medium format camera is the Hasselblad H2. I use a variety of medium format lenses from Hasselblad.
My preferred medium format digital back is the Phase One P45. This is a 39 megapixel back with stunning image quality. Capture One software by Phase One is used to "process" the RAW digital images from the Phase One back.
35mm SLR (Film) and DSLR (Digital SLR)
The SLR or Single Lens Reflex camera, whether film or digital, is the best format to use when shooting on the move or when shooting fast moving subjects (sports, animals) or when there are severe restrictions on how much equipment can be carried. The main advantages are simplicity of operation, powerful autofocus systems, advanced built-in metering, and ergonomic designs which result in being able to shoot quickly. The principle limitation of 35mm/DSLR is the smaller film or sensor size which reduces the size of enlargement that can be made for a given degree of sharpness. Still, images from current digital SLRs and 35mm film (when scanned properly on a high-end drum scanner) can be successfully enlarged to make beautiful prints.
My preferred 35mm film camera is a Nikon F6. My preferred DSLR is the Nikon D2X. Nikon's metering system is second to none and the new features of the F6 and D2X make it the perfect camera for fast moving animal photography and sports. I use a variety of Nikkor lenses.