Sampling is the digitization of the image in a X/Y coordinate manner, taking the image and representing it as a grid of pixels. The above image uses different keyboard keys to represent the pixels of the image.
Quantization is assigning an intensity value to these pixels and when these two concepts are combined we are able to accurately represent a continuous image digitally. In relation to the above picture, quantization would be how intense the color is on each of they keys in the above image are.
samplingexplained 