2.RANGE FINDER CAMERAS where in the view finder we see two images and we turn the ring of the lens and the image is focused when both the images in viewfinders overlap exactly at same place. 

3. SLR or SINGLE LENS REFLEX CAMERAS are the most important cameras in the category, which is actually what most of the photographers’ use and a serious photographer cannot go without it.
The major advantage of SLR cameras is that what we see in the viewfinder is what we get.
SLR cameras are available in market in manual and auto ones and one can choose according to his requirements. 

35mm cameras, even today are the most widely used cameras world over because of the kind of options we get with these cameras are huge. Manufacturers have pumped in billions of dollars in the research work for such cameras and the accessories available for these cameras and lenses options are just immense and you can get a camera, which suits your requirements 99.99% if not 100%.
In India alone the sale of 35mm film are about 90-95 million rolls per year. With this you can imagine the use of these cameras  

Then comes the next category of cameras, which we call MEDIUM FORMAT CAMERAS (medium format because the film size is in between the 35mm and large format 4X5 or 8X10). 

These cameras use the 120 films and most of the people in 60s and 70s were using these cameras before the advent of 35 mm and who didn’t like to use the bulky large format ones.
Today only professionals use the medium format cameras as these cameras are the entry point for a professional work and in fact most of the professional work is done on medium format cameras these days. 

Medium format cameras also available in range finder and reflex cameras. 

Earlier most of the cameras were TLR or TWIN LENS REFLEX where there used to be two lenses in the camera the lower one was called the taking lens with which film used to be exposed and the upper one is called the viewing lens with which we see the image
But the problem with TLR was parallax error in close situations. SLR medium format are the most widely medium format used today for work as they provide the exact view of what we are going to get on the film. 

These cameras use 120 films and the negative size we get varies from 6 X4.5 to 6 X18 cm. 

Medium format cameras are 90% of the times are manual operated but new advancements in technology have given birth to automatic medium formats like Mamiya 645 and Hassalblad H1. 

The most common formats are--- 

6 X 4.5(Mamiya 645,Hassalblad H1 etc)
6 X 6(all Hassalblads except Hassalblad X-PAN)
6 X 7(Mamiya RZ Mamiya RB,)
6 X 8(Fuji)
6 X 9 (Bronica)
6 X 18 (one of Fuji Panoramic) 

Hassalblads use 6 X 6 format where you get a square negative but the problem is that every time you will have to give instructions to lab for the croping of the negative. In printing, our prints are never in square format and the final result you get will be more or less what you will get from a 6 X 4.5 negative.
I use Mamiya RZ 67 pro 2 simply because of the reason that negative size I get is 6 X 7 and I don’t need to give instruction to lab every time for cropping and its much cheaper than Hassalblad price wise and I get nearly the same quality as hassy if not higher.
But the disadvantage is that it is much bulkier than hassy but it doesn’t matters to me much as most of the time I am on tripod.
Polaroid backs are also available for these cameras and you can judge your shot before actually exposing onto the film.
Also as digital technology is coming up fast there are also digital backs available for these cameras and are quite well also. 

Lastly there are LARGE FORMAT CAMERAS available that use film size of 4 X 5 or 8 X 10 inches. They are very bulky and are only used by professionals. But the quality you get from these cannot be beaten by any other format and you can blow up your negatives to huge hoarding size posters.
Major company that is making large formats is Sinar and even digital backs for large formats are also coming up to catch that market also. 

The most recent advancement in the field of photography started in early 90s when came the DIGITAL CAMERAS which had a light sensitive CCD or CMOS sensor build in to capture the light and recorded the light on pixels and not on film and we could see our image as soon as we shoot on LCD of the camera itself and also transfer it to computer for manipulation.
Digital photography opened up a completely new field of possibilities where one could do anything to image sitting on his PC for which we used to visit labs. 

Today the market is flooded with digital cameras and you will find a digital camera in hands from a holiday shooter or amateur to professional photographers.
Digital cameras have given a run of money to film cameras and most of the major manufacturers are now pooling up most of their research on digital cameras and even some of the companies like Olympus has totally stopped producing film cameras and concentrating only on digital cameras. 

Digital cameras are much easier to use than film cameras but has its disadvantages also.
The debate that which is better digital or film wont die at least for next couple of years but what I have learned by my personal experiences is that film is still the best as far as quality is concerned and also film cameras are never going die even if digital cameras reach 100 million pixels.