Quadrature?

So each night (almost) I read my email and this list. Somewhere between 2
and 7 days a week, someone invarably brings up encoders. Now for the most
part I understand the basics of encoders and dro's. But one of the things
that I'm a little fuzzy on is what Quadrature is. I was cruizing ebay and
ran across some encoders (4) that claim a line count of 2500 and 10,000 in
(here's that word again) Quadrature.

So, what is it? And how should I approach it when starting to set up a dro?

Thanks in advance,

Scot