Well, it's time once again to display my ignorance. I was reading a thread today where someone was asking for chords to a particular song and one of the chords that was given was a - D9.

I am self taught, don't read well enough to sight-read a tune on stage and really have little to no theory training, so... What is a
D9 chord? Is it any "D" chord with a 9th added, or is something in the "D" chord removed and the 9th added in its place? Where does the "9" come from - the "D" scale, perhaps?

These are things I should probably know, but don't; or, they are things that I know, but don't know that I know them - I've just been playing them and not knowing what they were. It just drives me nuts when I see chords written as a D9, or a D6 and I sit there asking myself - now what the hell is this? Clear me up folks. Thanks.