We've discussed digital camera lenses before in detail. Now it's time to talk about sensors. While lenses are extremely important in how the picture looks in the end, it's the sensor that actually collects the data that forms the picture. Digital camera sensors are much less varied, much less transparent (both literally and, for compact cameras, technically), and they certainly aren't interchangeable, but their characteristics are every bit as important to the final shot as the lens.  

For sensors, size means everything. 
 
The larger the sensor, the better the picture and the higher the resolution it can push while maintaining a good image quality. Both compact cameras and SLRs can easily reach 14 megapixels, but the difference in image quality, clarity, and noise at that resolution is remarkable. Compact cameras have very small sensors, so they tend to display a great deal of noise, especially at higher ISO settings. Digital SLRs have large sensors, so even at identical resolutions they manifest much less noise and the pictures tend to be sharper. 
 
Sensor size can vary even among digital SLRs. These cameras generally use either APS-C or full frame sensors. These names are taken from the film upon which the sensor sizes were based. APS-C is based on Kodak's Advanced Photo System "Classic" frame size, where the film frames measured 25.1 x 16.7 mm. APS-C sensors can vary by a few millimeters, but remain around those measurements. Full frame is based on 35mm film. A full frame camera's sensor is 35mm wide, the same size as a frame of 35mm film.  
 
These sensor sizes are important in digital SLRs not only for picture quality, but for lens choice. Non-full-frame cameras employ a "crop factor" when using lenses intended for 35mm cameras. APS-C cameras use a crop factor of 1.5 to 1.7, meaning a lens that's 50mm on a 35mm camera works the same as a 75 to 85mm-equivalent lens on an APS-C camera. Full frame SLRs, like Nikon's D3x and  Canon's 5D don't have a crop factor; a 50mm lens acts like a 50mm lens.  
 
Micro Four Thirds cameras sit between compacts and SLRs, with sensors measuring approximately 18 x 13.5mm. The sensors are still much larger than compact cameras, but don't even reach APS-C size. Their crop factor is 2x, meaning a 25mm lens on a Micro Four Thirds camera is equivalent to a 50mm lens on a full frame camera.  
 
On the other end, some professional photographers use much larger and more expensive cameras than even full frame SLRs. Medium format cameras, like those made by Hasselblad, use sensors much larger than full frame. Hasselblad's flagship, the H4D-60, uses a 40 x 53mm sensor and can produce photos up to 60 megapixels. Of course, Hasselblad's equipment is strictly for big-budgeted professionals; an H4D-60 costs around $30,000. A Nikon 3Dx, in comparison, costs a little over $7,000.