CCD- Charge Coupled Device.Its basically the sensor that detects
light coming in when you snap a picture.The lens opens, and the
light falls on this sensor, which is made up of millions of tiny
detectors.
3.1 and 4, denotes the number of pixel detectors, which translates
to the amount of detail that you can capture.The higher the
megapixel, the more detail, but it can alsomean more noise
too(since noise is also something part of detail).
Another definition I found on the web says
"Charge Coupled Device: one of the two main types of image sensors
used in digital cameras. When a picture is taken, the CCD is struck
by light coming through the camera's lens. Each of the thousands or
millions of tiny pixels that make up the CCD converts this light
into electrons. The number of electrons, usually described as the
pixel's accumulated charge, is measured, and then converted to a
digital value. This last step occurs outside the CCD, in a camera
component called an analog-to-digital converter" 
