Image of comparison between sensor sizes

A camera sensor will be a
CCD (Charge Coupled Device) or a
CMOS (Complementary metal-oxide semiconductor)

CCDs used to be the best sensor and were used in high-end cameras, CMOS sensors are cheaper, use less energy and are getting increasingly better and tend to be used in most modern cameras.

Sensors vary in size common sizes are Full Frame, APS-C and Micro 4/3rds. A full frame sensor is the same size as a 35mm film frame (Approx. 36mm X 24mm and is used as the standard for measuring sensor size.

The sensor size effects the effective focal length of the lens. For example a 50mm lens on a full frame camera has a crop factor of 1.6 or 1.5 on an APS or Crop size sensor, making the 50mm lens effectively approx. 75mm.

In turn this effects depth of field and depth of field is generally shallower with larger sensors.

The sensor is sensitive to dust, which will appear as dust spots on your images.
To avoid this as much as possible:
Don’t touch the camera sensor.
Try to shield it from the wind when you are changing lenses.
Switch off the camera when changing lenses.

For sensor cleaning:
APM Camera Repairs
Wex Photographic Newcastle