Wow, 5 write-ups and still no definition. So let me have a try (I'm not an economist nor a manager, so take this with a grain of salt):

If you grew up in a capitalistic country, you have probably learned at school that there are three factors needed in every economy to create anything of value (be it a product or a service): Labor, Land and Capital. Since capital (i.e. machines) is in the long run normally cheaper than people, much of the physical labor needed to produce things has already been replaced by capital (and some 'land' like e.g. energy). Now many workers contribute their knowledge and their experience instead of their strength. Knowledge management can be seen as an attempt to increase the efficiency of these knowledge workers similar to how the assembly line increased the efficiency of physical workers.

A lot of people mean different things when they say "knowledge management". In the sense of the broad definition given above knowledge management encompasses a lot of practices:

Of course computer scientists are often fascinated the most by the "Knowledge capture" part, but also management types seem to like it. Since knowledge is - in contrast to strength - closely related to its owner (i.e. the employee), companies fear that success-critical knowledge might get lost as employees are increasingly willing to change their employer. A strategy to defend against this (beside a lot of others) is to try to capture the knowledge of the workers in an easily accessible way so any replacement worker can start on nearly the same level as the went-away. Capturing knowledge already has happened for a long time. Just remember all the reports, papers, ... you or your boss have to write. But with the advent of computers, the web and full-text search indices the 'easy access' problem might also become solvable. One approach would be to have workers enter their findings into a database like E2 where others with the same information need can find them later on. (And then there are all these visions of Science-Fiction authors about information accessible with a thought and an eyelid wink.) Of course this creates up-front costs (the work time of entering knowledge) so don't expect it to happen soon in the current quarterly result-oriented economy.

In such a 'mechanistic' knowledge management scenario humans are in the long run only needed to process information using the knowledge provided by the system and producing new knowledge by having lapses of creativity. In the very long run the system itself might become intelligent enough to do all the processing on its own. That will be the time when Men is no longer needed.

As a contrast to this top-down definition have a look at "What is knowledge management" (http://www.media-access.com/whatis.html) an IMHO good bottom-up definition.

(BTW, I'm also not a native English speaker. Please /msg me if any of the economic terminology above is wrong.)