Semantic memory is the portion of long term memory which is concerned with ideas, meanings, and concepts which are not related to personal experiences. Together with episodic memory, it makes up the section of the long term memory known as declarative memory. The long term memory also includes procedural memory, which is the memory of how to do things. These three different kinds of long term memory all interact with each other to allow people to do everything from reading a book to flying a space shuttle.
This type of memory is concerned with independent facts, such as what a refrigerator is, along with concepts which may be more difficult to define. By contrast, episodic memory involves events of personal relevance, such as the name of the next door neighbor's child. An example of semantic memory would be a discussion with someone in which he or she mentions owning a cat. Rather than recalling a specific episodic memory of a cat, someone can pull up the semantic definition of a cat to understand what the other person is talking about.
It may take several exposures to an idea or concept for a definition to stick in the semantic memory. This type of memory can also become confused during the early stages of learning, as for example when someone struggles to understand that two radically different styles of chair are both considered chairs, while also grasping the difference between a chair and a bench. This type of memory is especially active in childhood, as children are constantly encountering new concepts which must be defined and filed away in the semantic memory.
Semantic memory also plays a role in many human activities. For example, procedural memory provides information about how to read a newspaper, but it is semantic memory which remembers what the different letters mean, and how they link together into words. It also allows a reader to understand written communications in multiple fonts, since the brain understands the concept of a letter, rather than a specific example of a letter.
People are constantly using the information stored away in their long term memory to deal with a wide variety of situations, from totally new situations like learning how to fly a plane to routine events like taking a shower. Damage to any part of the memory can cause confusion and distress, as the brain may have difficulty contextualizing an activity or event without assistance from the long term memory.