A long-term memory is anything you remember that happened more than a few minutes ago. Long-term memories can last for just a few days or for many years. Long-term memories aren't all of equal strength. Stronger memories enable you to recall an event, procedure or fact on demand — for example, that Paris is the capital of France. Weaker memories often come to mind only through prompting or reminding.
Long-term memory isn't static. You do not imprint a memory and leave it as if untouched. Instead, you may revise the memory over time — perhaps by merging it with another memory or incorporating into it what others tell you about the memory. As a result, your memories are not always reliable.
The two major subdivisions of this sort of memory are explicit memory (also called declarative memory) and implicit memory (nondeclarative memory).
It's the type of memory that doesn't require conscious thought. It enables us to carry out commonly learned tasks without consciously thinking about them. It's our "how to" knowledge — what allows us to ride a bike, tie a shoe and wash up the dishes after dinner. Even the things we think of as natural tasks, such as walking, require this kind of memory.
Retrieving an explicit memory requires conscious thought — such as recalling who came to dinner last night or naming animals that live in the rain forest. It's what most people have in mind when they think of "memory" and whether theirs is good or bad. Explicit memory is often associative; your brain links these kinds of memories together. For example, when you think of a word like "car," a whole host of associated memories may come to mind — from rearview mirrors to your commute to a family road trip.
There are two kinds of explicit memory: episodic and semantic.
Semantic memory accounts for our "textbook learning" or general knowledge about the world. It's what enables us to say, without knowing exactly when and where we learned it, that a zebra is a striped animal, or that Vermont is in New England. Scientists aren't sure where semantic memory happens; some say in the hippocampus and related areas, while others think it's widely spread throughout the brain. Semantic memory is better sustained over time than episodic memory. We are often able to retain a highly functioning semantic memory into our 60s — after which it begins slowly declining.
Episodic memory, by contrast, is autobiographical: It provides us with a crucial record of our personal experiences. It is our episodic memory that allows us to remember the trip we took to the British Virgin Islands, what we had for dinner last night or who told us that our friend Maryann was expecting. Any past event in which we played a part, and which we remember as an "episode" (a scene of events), constitutes an episodic memory.
This form of memory appears to be centered in the brain's hippocampus — with considerable help from the cerebral cortex. Read more about this type of autobiographical memory and take a test to see if your episodic memories center in the same time of life as the average person's.