CCD Analogy
A common analogy for the operation of a CCD is as follows:
An number of buckets (Pixels) are distributed across a field (Focal Plane of a telescope)
in a square array. The buckets are placed on top of a series of parallel conveyor belts and collect  rain  fall
(Photons) across the field. The conveyor belts are initially stationary, while the rain slowly fills the
buckets (During the course of the exposure). Once the rain stops (The camera shutter closes) the
conveyor belts start turning and transfer the buckets of rain , one by one , to a measuring cylinder
(Electronic Amplifier) at the corner of the field (at the corner of the CCD)
The animation in the following slides demonstrates how the conveyor belts work.