Today seem Dsrl and Milc cameras are more and more popular with interchangeable lenses. A lens usually only can be used within a Camera system. For example Canon lenses can be used in the Canon system, Nikon lenses in the Nikon system, Pentax lenses in the Pentax system, Sony lenses in the Sony system, Fuji lenses in the Fuji system and so on.
To understand lens system for needs to understand camera sensor sizes.
Large format size more than 4*5 inches, Medium format: biggest size, more than 36*24mm, but smaller than 4*5 inches, Full frame, Fx or Film: 36*24mm, Aps-C or Dx: app 22.5*15mm, Micro 4/3:18*13mm.
The most common formats today are the Aps-C format or Dx (most popular), Full frame or Fx, and micro 4/3
Aps-C format – interchangeable lenses
Perhaps the most popular format today, such as entry or mid level Canon and Nikon cameras, Pentax cameras, most Sony cameras, Fuji-X sensor cameras, Sony Nex, and Samsung advanced cameras has this format. The sensor size is app 22.5*15mm. The Nikon Dx cameras has little bigger sized sensor, then Canon cameras.
Full frame format – Interchangeable lenses
Full frame Format cameras are significantly more expensive than the Aps-C sized cameras. They have a better image quality especially in low-light because of the bigger more light pixels they have. They have a sensor size of 36*24mm as the size of the traditional film. Most Full frame sensored cameras are selled by Canon and Nikon.
Interchangeable lens systems
Canon Full frame and Aps-C and mirrorlens M
Nikon Fx and Dx and mirrorless Nikon 1
Sony A and E mount (formerly Nex)
Panasonic and Olympus Micro 4/3
The parts of the camera, where we attach the lens, we called mounts. The different manufacturers usually gives names for their mounts: Canon calls their mount EF, Nikon F, Pentax K, Sony alpha or A etc.
Autofocusing and manual focusing lenses
Most lenses manufactured today are autofocusing lenses, but few are manual focusing. There are lots of older lenses with manual focusing. Some of the older lenses are quite good optically.
Adapters between different mounts – Interchangeable lenses
In some cases we can use lenses on the other manufacturer’s cameras, but not everytime. For example we can use Nikon lenses on Canon cameras, but not the other way usually we cannot use Canon lenses on Nikon cameras.
Third party lenses
Some Manufacturer makes lenses for other camera systems. This manufacturers (Sigma, Tamron) usually reverse engineer the main manufacturers autofocus modules. This can lead to incompatibility with future camera bodies. Third party lenses are usually cheaper than the main manufacturers products, and some time very good bargains. I have a Tamron 17-50 lens for example, because Canons lens is 2x in price, but only has a little advantage in autofocus speed/quality. Everybody can decide risk the incompatibility or go for a good bargain. Not a big risk though, because not very much incompatibility problems known nowadays, the only thing is come to my mind that some lenses are incompatible with the newest Nikon d5300 camera. You can check the camera before buy it, if it is compatible with a specific lens or not.
Canon “EF” lens system
Canon EF mount
Canon has perhaps the most lenses available in the camera market and has a biggest market share. Canon has EF, and EF-s lenses for their Dsrl cameras, and a different mount for the Eos-M mirrorless system. What are the differences? The EF lenses are compatible with Full frame cameras, the EF-s lenses only compatible with the smaller sensored Aps-C cameras. The Ef-s lenses cannot even be attached to the Full frame cameras unless you cut the usually plastic back part of the lens. Obviously Canon doesn’t like the idea to choose cheaper, but sometimes very sharp lenses on a Full frame camera, buy much more expensive Full frame lenses instead. Non-Canon (Sigma, Tamron, Samyang or called third party) Aps-C coverage (smaller) lenses can attach to Full frame cameras usually without problem, but obviously there will be black frame, or at least darker corners around the lens because of the smaller sensor coverage. Autofocus, metering also works normally. The Mirrorless Canon is a different system, because here the distance between the sensor plane is much smaller. Normal Canon lenses not fit to Eos-M system, unless with a special adapter. Other way around Eos-M lenses are not good for Dsrl cameras, because it is not possible to put the lens so close to the sensor. Canon make a new mount for Digital cameras, which means older Canon lenses cannot be attached to new Canon digital bodies, just with a very hardly can get, magnifying adapter, which is not preferred anyway. But older Canon lenses can be attached to other systems like Micro 4/3 for example, but with restricted functions (no autofocus and perhaps aperture control).
Nikon “F” mount lens system
Nikon is the second biggest player with lots of lenses, practically the same amount of lenses as Canon. Nikon has an F-mount with Fx(Full frame) or Dx(Aps-C) lenses and a mirrless Nikon 1 mount, which is much smaller sensored camera system. “Normal” Nikon lenses can be attached to the Nikon 1 system with a special not cheap adapter. Other way no meaning, and works either because the Nikon 1 lenses are very small for a Nikon dsrl camera. Nikon keep it’s old F mount, so in theory all old Nikon lenses can be used on digital cameras. But entrly level cameras like Nikon d3xxx, or Nikon d5xxx cameras lacks in built-focus motor, and with older manual focus lenses metering is also not guranteed. So if you plan to use older lenses perhaps must buy at least Nikon d7000, Nikond 7100 level camera to be able to do it. Dx lenses can be used on newer Full frame Nikon cameras, which usually have a “Crop mode” especially made for this reason.
Sony or Minolta “A” and “E” mount lens system
Sony or previously Minolta has two mount the “A” and “E” mounts. The “E” mount is a mirrorless cameras before called Sony Nex cameras, smaller cameras with Aps-C sized sensors. The “A” mount lenses are for Sony dsrl system. Sony use translucent mirror which is their unique product, these cameras has an “SLT” in their names. Usually older Minolta lenses can also be used on Sony cameras. Sony also has a unique Full frame mirrorless camera system with a different mount.
Pentax “K” mount lens system
Pentax is an older manufacturer with many old and new lenses, but the variety and the quality is not up to Canon’s and Nikon’s. Pentax has a mount named “K” mount.
Micro 4/3 lens system (Panasonic, Olympus)
The micro 4/3 lens system is newer than Canon, Nikon or Pentax systems. The micro 4/3 is a smaller sized mirrorless system. Panasonic and Olympus makes cameras and lenses in this system. The smaller sensor size and lack of mirror helps the lenses and cameras to be smaller, lighter and thinner. The autofocusing in video recording is better(Spring 2014) than the original dsrl systems, which doesn’t designed originally for this reason.
Fuji “x” mount lens system
Fuji x-mount is perhaps the newest today. Fuji invented a very interesting new sensor with a very good high iso capability. Currently not so much lenses are available, but the list is growing. The sensor used in Fuji-X cameras are Aps-C sized.