Hi Shady,
Dropped tunings refer to taking a regular tuning and lowering just 1 (or sometimes 2) of the strings. In the case of Dropped-D, it's the same as standard tuning (EADGBE) but has the low E string lowered to a D so that you get DADGBE. There are other drop-tunings out there, but this is the most common one.
Drop-tuning is NOT the same as "tuning-down" or "downtuning" or whatever, which is lowering ALL strings the same amount. Most common example of this is tuning down half a step. Among many others, Jimi Hendrix is one who did this a lot. Basicly, you take standard tuning (EADGBE) and lower ALL strings half a tone so you get Eb,Ab,Db,Gb,Bb,Eb.
As for modes, try
this link. It's all pretty understandable in my eyes, but for the essence, stick to page 3 and 4. These explain the relative modes (the modes you get by taking a different "tonal center" of the same scale each time) and parallel modes (the different modes used on a same "root note") and the way they are constructed in a pretty straightforward manner. Hope that helps
