Quantum Efficiency (QE)
Quantum efficiency is the number of photoelectrons that are generated in a pixel for every photon of light that hits the surface above it. Quantum efficiency is, in simple terms, a percentage of light you detect with your CCD. Silicon can see wavelengths in the range of 200 nm to 1200 nm. This is 2-3 times the color range of the human eye! The human eye only has a QE of about 10%, but the QE of CCDs can approach 80%. For a perfect device, where every photon generates a photoelectron, the quantum efficiency is 100%. In practice, the quantum efficiency will be less than 100% in all places since there are unavoidable losses. Some of this is the result of the atomic structure of silicon, and some because light must pass through a silicon surface to get into the CCD to generate a photoelectron. There are many tricks for increasing the quantum efficiency. For a CCD made from bare silicon, the quantum efficiency is still very good! :: More information on QE ::

Charge Transfer Efficiency (CTE)
Charge transfer efficiency is how well the photoelectrons from one pixel are transferred to the adjacent pixel during a shift operation. If the efficiency is 1, then all the photoelectrons are always transferred without any loss. In the water bucket analogy, this is equivalent to how much water is lost between the buckets. Normal charge transfer efficiencies are 0.99999 to 0.999999, meaning that one photoelectron is lost for every 100000 to 1000000 shifts … very impressive indeed! 2000-4000 shifts are needed to read the CCD, so a CTE this good is necessary. For example, if the CTE was only 0.999, you couldn't read most of the CCD. CCDs that have a very low CTE will leave streaks which are caused by charge/electrons being left behind after a transfer.

Dark Current
Even in the absence of light, some electrons will accumulate in the CCD pixels -- this is called "dark current." These electrons are not generated by incoming photons, but are randomly generated by thermal excitation -- the random motions due to the temperature. Thus, this effect is greatly reduced by cooling the CCD. Astronomical CCDs typically operate at -90 C to -40 C. Because the dark current electrons are randomly generated, they add to the noise of a measurement, so this is an important effect to minimize.

Read-out noise
Read-out noise occurs when the photoelectrons are converted to a voltage. The electronic amplifiers that do this are not perfect, so they introduce a “noise”, or uncertainty, in the measurement. Typically, each read-out has an uncertainty of between 1 to 10 electrons, depending on the CCD and how it is operated. Read-out noise is one of the limitations on how faint an object a CCD can detect.