Memory is the process of maintaining information over time through encoding, storage, and retrieval. It allows us to draw upon past experiences. There are three main types of memory: sensory memory, which briefly stores sensory information; short-term memory, which can hold information for around 20-30 seconds; and long-term memory, which stores information for longer periods ranging from minutes to a lifetime. Factors like meaningfulness of information, amount of learning, method of learning, age, health, and intelligence influence how much information is retained in memory. Forgetting occurs through processes like trace decay, interference, and failure of retrieval.