Good advice: Read the manual!
The question you've asked are basic knowledge if you intend to work with the Virus.
Anyway:
SINGLE is where you create patches if you only use the Virus as a SINGLE-patch instrument.
MULTI: You can see this as a bank of instruments, but you can also use this mode to layer sounds(which I guess is it's primary function).
SINGLE-MULTI: Is what you must use if you want to play different patches on different midi-channels.
Notice that in both Multi an single-multi mode, you can only have ONE delay/reverb effect for the whole collection (that somehow could make sense in multi-mode, but absolutely not in single-multi mode!)
Now, go read the manual...it's REALLY worth it!