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storage - the retention of information;

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retrieval - the recovery of stored information.
It is possible to distinguish between three forms of memory:

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ultra short-term memory (or sensory storage);

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short term memory (often referred to as working memory);

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long term memory.

Ultra short-term memory has already been described when examining the role of sensory stores. It has a duration of up to 2 seconds (depending on the sense) and is used as a buffer, giving us time to attend to sensory input.
Short term memory receives a proportion of the information received into sensory stores, and allows us to store information long enough to use it (hence the idea of ‘working memory’). It can store only a relatively small amount of information at one time, i.e. 5 to 9 (often referred to as 7 ±2) items of information, for a short duration, typically 10 to 20 seconds. As the following example shows, capacity of short term memory can be enhanced by splitting information in to ‘chunks’ (a group of related items).
A telephone number, e.g. 02086466951, can be stored as 11 discrete digits, in which case it is unlikely to be remembered. Alternatively, it can be stored in chunks of related information, e.g. in the UK, 0208 may be stored as one chunk, 648 as another, and 6951 as another, using only 3 chunks and therefore, more likely to be remembered. In mainland Europe, the same telephone number would probably be stored as 02 08 64 86 95 1, using 6 chunks. The size of the chunk will be determined by the individual's familiarity with the information (based on prior experience and context), thus in this example, a person from the UK might recognise 0208 as the code for London, but a person from mainland Europe might not.
This duration can be extended through rehearsal (mental repetition of the information) or encoding the information in some meaningful manner (e.g. associating it with something as in the example above).
The capacity of long-term memory appears to be unlimited. It is used to store information that is not currently being used, including:
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knowledge of the physical world and objects within it and how these behave;

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beliefs about people, social norms, values, etc.;

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motor programmes, problem solving skills and plans for achieving various activities;

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abilities, such as language comprehension.

Information in long-term memory can be divided into two types: (i) semantic and (ii) episodic. Semantic memory refers to our store of general, factual knowledge about the world, such as concepts, rules, one’s own language, etc. It is information that is not tied to where and when the knowledge was originally acquired. Episodic memory refers to memory of specific events, such as our past experiences (including people, events and objects). We can usually place these things within a certain context. It is believed that episodic memory is heavily influenced by a person’s expectations of what should have happened, thus two people’s recollection of the same event can differ.
　
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