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.

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