Semantic memory consists of your general knowledge about the
world. For example, you know what a poodle is, you know that it's a
kind of dog, that beagles and rottweilers are
other kinds of dogs, and that dogs can be trained, but nonetheless tend to
poop in inconvenient places.
Now, you probably don't remember where you learned all of that, but you
know it anyway. According to Endel Tulving, who first came up with the
distinction, that's a defining feature of semantic memory--it's something
you know, even though you can't remember where or when you learned it.
(If you have a memory about a particular pooping dog, or your
best friend's rottweiler, or anything else that involves a specific event,
then that's an episodic memory.)
The neurobiology of semantic memory remains unclear, but some recent
evidence suggests that the lateral temporal lobes play an
important role. In a rare disorder called semantic dementia, this
brain region degenerates, and patients slowly lose their knowledge about
the world. One patient, for example, no longer knew anything about teapots; another couldn't figure out what to do with a
clothespin.
Semantic-dementia patients are sometimes mistaken for visual
agnosics, but there's an important distinction between the two.
Patients with visual agnosia cannot recognize a teapot by sight, but they
can usually answer properly if you ask them how one uses a "teapot," and
they may be able to make tea without difficulty. A semantic dementia
patient, on the other hand, can neither recognize the teapot nor explain
its function.
Also, aphasic or anomic patients don't necessarily
have problems with semantic memory. Aphasics often lose the ability to
think of specific words, but they often know what they're trying
to say (just as you do when you say "Give me the...uh...uh...the THING,
damnit! You know what I mean!). In the example above, an aphasic might
not be able to remember the word "teapot," but would still know what it was and how to use it; he would know everything about it except its
name. | 
