Faceless, to clarify:
A guitar has a standard tuning of E A D G B E..... which is, lets call it the "base" tuning.
1) drop tuning
Drop tuning means that you "drop" the last string to a full step down, but don't touch the rest of the strings..... which reads as D A D G B E.
The reason for this, is easier fretting for bar chords, and also giving you a deeper, lower tone (normally for metal) while still able to retain the standard tuning for solos and stuff like that.
For example: To play a G minor Chord on standard tuning, you'd fret:
E---3---
A---5---
D---5---
G---3---
B---3---
E---3---
That means you have to bar the chord with your index, ring and pinky finger. But if you Drop D
D---5---
A---5---
D---5---
G---x---
B---x---
E---x---
See? Because the E is now Dropped to a D, I can just bar with my Index finger. It's the same chord, but I just need one finger to barr the notes.
2) flat tuning
Means that instead of tuning only 1 string down, you tune all the strings down from the "base" tuning. Which means if I want to tune to a D, I have to tune E A D G B E all one tone down, or one full step down: D G C F A D.
But you can also tune it a "half step down", meaning you can just tune it to Eb Ab Db Gb Bb Eb. This is equal to playing everything one fret down from standard. A full step down means everything is played 2 frets down from standard.
3) Standard Tuning are of course the standard E A D G B E base tuning.
More info can be found here:
http://en.wikipedia.org/wiki/Guitar_Tuning