Human memory involves an encoding process used to transform information to be stored for later use, similar to encoding on a computer. We take information and transform it into something meaningful such as "an association with an existing memory, an image, or a sound." After this encoding process, the brain stores the information to hold onto via a physiological change in the memory. Finally, after memories have been stored we retrieve that memory and reverse the encoding process. "In other words, return the information to a form similar to what we stored."
Sensory memory involves what we take in via the senses, lasting only a few seconds. When this sensory memory moves to our awareness, it becomes short term memory. Lasting longer than sensory memory, short term memory can last up to thirty seconds. Once the short term memory is full, displacement occurs pushing out the old information in order to make room for the new. Long term memory, which is similar to the permanent storage of a computer, is "relatively permanent and practically unlimited in terms of its storage capacity." Unfortunately we don't even use a fraction of the space available for storage within our long term memories. The long term memory is divided into a few subcategories: declarative memory, semantic memory, episodic memory, and non-declarative memory. These include life events, factual knowledge, and memories obtained through extensive practice or habits.
As much as we remember, we forget. Forgetting is a natural phenomenon for which there are many reasons. Sometimes, information either never makes it to the long term memory or it does but it is lost before it can attach itself there permanently. Forgetting also occurs when information is not used for an extended period of time and it literally fades away. "Failing to remember something doesn't mean the information is gone forever though. Sometimes the information is there but for various reasons we can't access it." Repression also causes people to forget as well as amnesia.