The sensor is an important part of your camera. And while it should never be the “be all and end all” when making a camera purchase, you should put thought and consideration into the type and size.
The CCD (charged-coupled device) is the most common type of sensor in a digital SLR. Every manufacturer offers at least one model with a CCD. They offer the highest image quality, hands down, but they are of course, the most expensive and use a lot of power.
CMOS (complementary metal-oxide semiconductor) strip away extra circuits on the chip to increase a pixel's light-collecting area while reducing costs and using less power than a CCD. The only con is they are bigger and therefore, the cameras are bigger.
What about sensor size? There are three standard sizes. The first is called Four Thirds, found on Olympus and Panasonic cameras. It's a standard size that was created by Olympus and Kodak, measuring 17.3mm by 13mm. Most other DSLRs use an APS sized sensor, the second standard size. It is a fairly loose term for a sensor the size of an APS-C or APS-H film format. Finally, we have the 35mm-film format, also called a full-frame because it is the size of a standard frame on a roll of 35mm film. These sensors are big and expensive to build. You'll find them on the Canon EOS 5D and the Nikon D3.
We can thank large sensors for giving us better photos with less noise, a greater dynamic range, and better performance at high ISO settings.