Sensor size

Let's look first at the physical size of CCD or CMOS sensor used to capture your photographs. Although there are slight variations, virtually all DSLR sensors fit into one of three size categories (starting with the largest); Full Frame, APS-C and Four-Thirds. Sensor size isn't as important as some people would have you think, but there are some key differences. The first is what is known as the 'crop factor'. As the diagram below shows, as the sensor gets smaller it captures a smaller area of the scene, resulting in a photograph that looks like it was taken at a longer focal length (1.5x or 1.6x longer for APS-C, 2x for Four-Thirds).
The crop factor isn't a major issue for most users, but it does have important consequences for some. If you're buying a digital SLR to replace a film model because you've got a kit bag full of lenses you need to be aware that unless you buy a full frame model all your lenses will produce very different results on your new camera.
For telephoto shooters the result is quite a bonus, as all your lenses will effectively get even more powerful. On the other hand the crop factor means your wideangle lenses will no longer offer anything like a 'wide' field of view. Fortunately there is a wide range of specially designed 'digital only' lenses for smaller sensor DSLRs.

The three most common sensor sizes compared: full frame, APS-C and Four-thirds. Smaller sensors 'crop' the scene and make a lens appear to have a longer focal length.	For sports and wildlife shooters the smaller sensor has the effect of making their telephoto lenses and zooms even more powerful.

So which is right for you? Each has its own benefits and each has its limitations, and if you're building a DSLR system from scratch you needn't get too hung up on which is right for you.
The largest (and most expensive) DLSR format is full frame (so called because the sensor is the same size as a frame of 35mm film). DSLRs with full frame sensors have the biggest, brightest viewfinders and because there's no crop factor are often chosen by photographers who are upgrading from a film SLR and already own expensive wideangle lenses. The larger sensor also means that - all other things being equal - full frame cameras will produce the best results in very low light and at higher sensitivities. On the downside, full frame cameras are big and expensive, and there is only a handful of models to choose from. You also lose the focal length 'boost' offered by smaller sensor cameras when shooting with telephotos.
APS-C is by far the most common format, used in virtually all Canon, Nikon, Pentax and Sony DLSR models. With a crop factor of 1.5x or 1.6x you need special digital lenses to get true wideangle results, but these are readily available and are usually less expensive than their 'full frame' counterparts. The 'kit lens' supplied with most APS-C cameras is a good starting point, offering a versatile zoom range from wideangle to short telephoto.

The difference in size and weight between a full frame professional level camera (Nikon D3, left) and a compact entry-level DSLR (Nikon D60, right) is considerable.	The Four Thirds system promises slightly more compact camera bodies and lenses. Four thirds is also the only DSLR system that uses the slightly squarer '4:3' format (as used on virtually all compact digital cameras).

Four Thirds is a new 'all digital' format developed by Olympus and currently used in Olympus and Panasonic DSLR models. Unlike the other systems on the market Four Thirds is not based on any existing film SLR system and uses a totally new lens mount, so all the lenses in the system are designed for digital, making the crop factor issues mentioned above less relevant. With the smallest sensor size Four Thirds offers slightly more compact camera bodies and lenses. Although the smaller sensor should in theory mean that these cameras produce noisier (grainier) results in low light and at higher sensitivities, for most purposes the difference isn't huge.