Newbie question on indexes

What exactly do indexes do and why do I want to use them? From my reading I  gather that it improves query performances somehow. Digging through my manuals  tells me how to create indexes, but can't find anything on the strategy behind  it. Also, if I create an index, are there any other settings that I need to  make, or do the indexes get used automatically somehow? Thanks - SS Received on Sat Oct 18 1997 - 00:00:00 CDT