CCDs comprise photodiodes and mechanisms consisting of polysilicon gates for transferring the charges accumulating within them to the edge of the CCD. The charges themselves cannot be read as electrical signals, and need to be transferred across the CCD to the edge, where they are converted into voltage. By applying series of pulses, the charges accumulated at each photodiode are relayed in succession, much like buckets of water in a bucket line, down the rows of photodiodes to the edge. CCD is the abbreviation of "Charge-Coupled Device," and "Charge-Coupled" means the way charges are moved through gates from one photodiode to the next.