There's a bunch of things with MF to know. I'm gonna have a crack and someone else can add more!
Film size: Referred to in cm e.g. 6x4.5, 6x6, 6x7, 6x9 Some cameras have masking capability allowing a variety of sizes, some don't.
Camera types: Most MF cameras are fully manual, some have light meters, and a small number have fully auto features. Most, however, are manual. Folders have a bellows and can be fitted in a normal big jacket pocket. These are generally very old and not suitable for wedding work as they are percieved as wierd or toys. SLRs are the square jobbies which have a BIG mirror which goes kerthunk. TLRs have two lenses - a viewing one and a taking one. SLRs generally have interchangeable lenses, cos what you see is what you get.
Viewfinders: waist level (useful for hand-holding and wow a big viewfinder), or prism (like a normal 35mm SLR). Usually the prisms have a basic meter in them and cost a fortune. Prisms are handy if you're using a tripod - as you can see the picture without stepladders.
Backs: Most SLR MF cameras have interchangeable backs. There's what's called a dark slide, which is like a sheet of metal which seals the light out. Then there are two reels one with unspent film and the other with spent.
The most popular setup is probably a 6x4.5cm mask on an SLR with a waist level finder. It's called 645 w/level finder in adverts. Most of the big MF manufacturers have one of these on offer. It's not too far from the standard aspect ratio, leaving you a moderate crop. 6x6 if you think about it, is square and therefore you're going to lose a bit of the frame. However, it doesn't matter whether the camera is vertical or horizontal (maybe a consideration with weddings)
All the mainstream MF cameras take 120 roll film, which you can buy in colour negative (easy) or colour slide (slightly more tricky) or B&W and other variations. I personally find Fuji's 100 negative film the best and easiest to use as it has a massive exposure latitude allowing for major metering screw-ups.
The Hassleblad 500cm with a good light meter is very hard to beat and you can get 645 or 66 backs for it and this makes it very flexible.
http://cgi.ebay.com/Hasselblad-500-...7578359911QQcategoryZ3351QQrdZ1QQcmdZViewItem
http://cgi.ebay.com/Hasselblad-500-...7579541684QQcategoryZ3351QQrdZ1QQcmdZViewItem
http://cgi.ebay.com/HASSELBLAD-500-...7579205271QQcategoryZ3351QQrdZ1QQcmdZViewItem
Look very closely at them as they can be had cheaply, but they may have been
battered by studio work. Make sure you ask about all shutter speeds working and that it's lightproof and fully functional with no brassing, fungus or wear to the lens mechanism.
Anything else?
Rob