Cellular principles of learning and memory

The tasks of the human brain, as well as the animal brain, are fairly complex: On the one hand an uninterrupted stream of sensory input has to be processed, on the other hand at the same time memories have to be stored, sometimes for a lifetime, and retrieved. The generation and storage of new information-codes, as well as the transmission of chemical messengers between neurons occurs at the synapses. But what are the cellular and biochemical mechanisms of learning and memory?