Reverb is a stream of reflections that are more or less delayed with respect to the direct sound.

Early reflections reflections arriving up to about 25 ms after the direct sound will most likely produce comb filtering, beneficial for bass instruments but ineligible for most of the others.
00-10ms - change in tonal coloration
10-20ms - area of maximal Haas effect
20-40ms - tail of the Haas effect, most of the early reflections should be right here
40-100ms - late reflections should be diffused

Room: Volume of low frequencies and master volume level of this reverb

RoomHF: Volume of high frequencies, controls brightness of the reverb and is very dependent on the Quality setting.

RoomRolloffFactor: Has no effect in Modplug.

DecayTime: defined as a time (in seconds) the reverb level at HFReference drops by 60 db. Concert halls have about 1.5-2 sec decay at 4khz.

DecayHFRatio: Rate between low and high frequency decay. Human year is supposed to notice a +/- 10% imperfection in a concert hall. Use low values for a special effect

Reflections: Early reflections volume level.

ReflectionsDelay: Early reflections delay (in miliseconds)

Reverb: Late reverbation volume level. Late reflections are more silent than early echoes but arrive in much higher amounts and for a longer period of time. They're the basic parameter which defines the character of the reverb.

ReverbDelay: Late reverbation delay (in miliseconds, adds to ReflectionsDelay)

Diffusion: Controls volume of random (well, not completely - their tap to tap distance depends on corellated reflection tap to tap distance contolled by Density) uncorellated reflections in reverb. Maximum value sounds artificial, something between 70 and 90 should work. If you want a room ambience (down to about 40) or a special effect, go lower.

Density: Distance between reverb reflections, the higher the value, the lower the distance. Use low values for a special effect.

HFReference: High frequency absorption reference frequency.

Quality: 3 = 44khz processing, 2 = 22khz and so on. The highest quality doesn't always give the best results, especially if you want to get rid of high frequensies.

Setting examples:

Concert hall

Room: -800
RoomHF: -800
RoomRolloffFactor: N/A
DecayTime: 2
DecayHFRatio: 0.9
Reflections: -1700
ReflectionsDelay: 0.15
Reverb: 200
ReverbDelay: 0.05
Diffusion: 80
Density: 100
HFReference: 5000
Quality: 1