Explicit memory, often referred to simply as conscious memory or declarative memory, is a form of recollection in which one makes a conscious effort to recall a particular piece of information. Implicit memory, in contrast, is a form of memory that positively affects current experiences based on lessons learned in past experience. Walking, for instance, is an action that relies on implicit memory; it is not necessary to consciously recall how to take steps in order to walk. Examples of explicit memory, on the other hand, include recalling a particular experience, such as a party, or recalling a particular fact, such as a person's name.
There are two different types of explicit memory: episodic and semantic. Episodic memory is the recollection of experiences and events, while semantic memory is the recollection of facts and other general knowledge. Episodic memory is often referred to autobiographical memory; it serves as a record of events or episodes occurring in one's life. After a period of "childhood amnesia" that occurs during the first few years of one's life, many people, even the very old, find that they can recall many events from their adolescent and young adult years with great clarity. After that, though, recent memories are generally recalled much more easily than more distant memories.
Semantic memory is not necessarily linked to any particular time in an individual's life. The facts that make up the semantic memory often are not connected to the specific events that resulted in the acquisition of the particular pieces of knowledge. It is still a form of explicit memory, though, as conscious effort is needed to recall the particular piece of information.
Both forms of explicit memory are often recalled through association. Thinking about a particular piece of information often results in the recall of many related pieces of information or even an episodic memory of where that information was acquired. Likewise, an episodic memory could result in the recall of semantic memories, such as names or dates.
Explicit memory often degrades over time. As people age, they lose the ability to recall events and facts with the speed and clarity of their youth. Sometimes, this is simply a result of the degradation that naturally occurs as a result of aging. In other cases, though, memory problems can be caused by neurodegenerative illnesses such as Alzheimer's disease. In particularly severe cases, even recent memories may be jumbled or entirely nonexistent.