Human memory is a complex process, in which several different areas of the brain act in conjunction with one another, and with many distinct categories that can be recognized. An elementary distinction is the dichotomy between short-term memory and long-term memory. The former refers to relatively unimportant information that does not need to be retained for very long – for example, what color shirt a coworker wore yesterday, or how many empty seats there were on the bus this morning. The latter refers to information that is relatively essential to retain indefinitely – for example, how many children you have, or where you were raised.
Another important dichotomy distinguishes between explicit memory and implicit memory. Explicit memory involves memories of facts and events, as well as your awareness of knowing these things. For example, you know that you are human, that you do not have classes on Saturdays, and that your brother is taller than you. Implicit memory involves memories of things like skills and habits that you are not consciously aware of knowing. For example, ordinarily, you do not consciously remember how to tie your shoes, or throw a ball, or drive a car. There is often some interplay between explicit and implicit memory, with explicit memories becoming implicit over time, or through practice and repetition.
The brain’s complicated storage and retrieval system distributes memories across several structures. One crucial brain structure is the cerebral cortex, the folded and convoluted outermost surface, where most shortterm memories are stored. Another critical structure is the hippocampus, which, while not a storage area itself, plays a vital role in transforming short-term memories into long-term memories. Without the hippocampus, no long-term memories can be made. Long-term memories themselves are maintained by stable connections of specialized cells called neurons, an estimated 100 billion of which form intricately woven networks throughout the brain.