The Mamiya C330 is a twin lens reflex medium format camera that was manufactured from 1969 to 1974. It is part of the Mamiya "C" line of cameras. It uses 120 or 220 film, but can also be refitted to accept sheet film.
Like all cameras in the Mamiya C line, it uses interchangeable lenses that are removed and attached by a metal clip on the front of the camera. On the C330, there is a knob on the left side of the camera that can be turned to lock or unlock. Turning the knob to unlock flips a cover up over the film chamber so any film in the camera isn't exposed when you remove the lens. It also releases a small metal piece that normally prevents you from accidentally disengaging the clip.
C330's generally come with a waist level finder and a generic screen. Both the finder and the screen are interchangeable. There are a variety of different focusing screens and finders available for the C330 since, by and large, the accessories for earlier and later models will all work.
Focusing is done with the ever-so-fancy "rack and pinion focus bellows system". The face plate that holds the lens is connected to the main body of the camera by a metal framework that is moved forward and backward with the focusing knob and protected from light with bellows. This is a big selling point, since is means that the camera has built in variable extension. In other words, you can focus on things very close to the lens. As the bellows are extended past a certain point, you need to compensate for the loss of light by modifying your exposure. In the finder, (assuming you have the appropriate mask on your finder for your lens) a little wire will descend through the field of view which indicates both where the top of the frame will be, as well as what you should multiply your exposure by. On the left side, there is a ring around the lens lock that lets you set which lens is attached, which actually just adjusts how the wire moves. With the bellows fully extended with a 80mm lens, you can focus around 8 inches to a foot in front of the camera (though to get an appropriate exposure, you need to compensate by about 3 stops.)
There are two shutter releases. One is on the lower right corner of the front (which accepts a cable release.) The other is a plastic thumb tab on the right side of the front plate. Both of these can be locked by moving a small sliding switch above the plastic tab to "L". On the right side of the camera are a crank and a dial. The dial sets the camera in either "single" or "multi" mode. In single mode, you must wind past the current frame before triggering the shutter again. Multi lets you take multiple exposures. In single mode, the crank advances the film as well as cocking the shutter. In multi mode, the crank just recocks the shutter.
It also includes a cold shoe flash mount and supports both electronic flash and flash bulbs (via the M/X selector above the taking lense.)
Switching between 120 and 220 film is done by rotating the pressure plate that the film travels across. The frame counter is able to deal with this automatically.
A few words on purchasing...
If you're thinking about purchasing one of these, I presume you already know why you want it and why you want to shoot medium format. So I won't waste a bunch of space explaining how bulky, heavy and hard to use it is.
In terms of the body, there are a few things that you'll want to check. If you're buying on-line or at a used camera store, make sure that you can return it if it's flawed. Don't buy these things "as-is, no return." Trust me, if they're squeamish about you returning it for defects, there's probably a good reason. The bodies tend to have two main problems that can't be checked just by inspecting it. One is whether the bellows are light tight or not. If you're shooting a test roll, make sure that you take at least one frame outside in full sun with the bellows fully extended. If the light leak is near the body or tiny, this should ferret it out, which is better than finding out after you've started using the camera for real. The other problem is how the film advances. Make sure that the gaps between frames are fairly regularly sized. You want to be getting your 12 exposures and you can also get an indication if the film advance mechanism is going.
In terms of buying lenses, your main tool is your ear. Sure, check out the glass, but the number one cause of lens death is the shutter failing. And the shutter usually starts failing at the slower shutter speeds first. Take the lens and set the shutter speed to the highest speed. Cock the lens using the metal cocking level on the right side. Trigger is using the manual trigger on the left hand side. Decrease the speed and do it again. When you get down to around 1/15th, you'll start being able to hear the mechanism. When you get down to 1/2 and 1, you'll be able to hear it clearly and well. The noise the shutter makes should be nice and even. There should be no "jerking" involved. If the 1 second or half second speeds sound like they're catching and uneven, don't buy the lens.
Sources:
The Mamiya TLR Faq at http://www.btinternet.com/~g.a.patterson/mfaq/m_faq-contents.html
Speaking with various Mamiya users
The shutter sound trick came from John Kimmich Javier
Playing with my beat up C330
Oh, and here's a picture...http://medfmt.8k.com/mf/kb/mamiya_c330f.jpg