What is a digital sensor?

As we discussed last time, a digital sensor and memory are what separate a DSLR from prior film-era SLRs. A digital sensor (or "image sensor") is an electronic device that converts light it senses into a digital signal.

Initially, sensors were charge-coupled devices ("CCDs"), but many if not most sensors are now complementary metal-oxide-semiconductors ("CMOS"). It used to be the case that CCDs produced superior images. However, manufacturers have poured much more money into CMOS R&D over the last decade or longer because CMOS-based devices are cheaper to make and require much less circuitry. As a result, CMOS sensors rival the performance of CCDs and today may even be better.

Just as with lenses, you need to keep your sensor clean. While that's not so hard with point-and-shoot cameras because they are sealed units, your sensor will likely get dirt on it over time if you change lenses.

The sensor (or, to be more precise, the low pass filter on top of the sensor) must be kept clean so that you don't have distracting blotches from accumulated dirt. Some cameras can clean their sensor automatically, but most require occasional manual cleaning. You can either clean the sensor yourself using available kits or you can have it professionally done. There's even a whole web site dedicated to sensor cleaning by a camera cleaning outfit. Note, however, that you should make sure that you are using the right equipment for the job. DO NOT use the compressed air cans -- you can damage your camera severely if you do, and are likely to introduce moisture.

Links:
1) Wikipedia article