According to psychological theory, memory, as a function of the mind, can be broken down into three separate functions. All of them are necessary in order for memory to work.
The first function is called encoding. In psychology, it is sometimes useful to compare the brain's functions to a computer. Information is received through the senses and then translated into a code that can be kept in the mind. It will be useful and comprehensible for the brain at a later date. As automatic as many of the brain's functions may seem in terms of experience, these steps must occur much like they would in a computer.
The second function is storage. This function may seem straightforward in that this is the task of taking the information that has been translated into code and finding a place where it can stored until needed at a later time. However, this function is not necessarily a simple one. Stored information must be found again. Therefore, a high level of organization is required. Memories are stored in what is sometimes called a "lexicon", which is, in essence, a giant web of interconnections. Memories are stored in terms of similarities to other information or details that they shares with other memories. These connections of meaning are important in order for the third function to be possible.
The third function of memory is retrieval. Once information has been translated to code, stored in its proper place within the lexicon, it still must be brought back into awareness in order for the memory process to be complete. In simple terms, it must be remembered, which means retrieving it from its storage place. Research has demonstrated that it is often this third function of retrieval that is most associated with memory loss. The information may be in the lexicon somewhere, but when a person cannot find memories that problems most often occur.