Read here:

http://en.wikipedia.org/wiki/Vocal_range

Basically you have: (if middle C = C4)

Soprano: C4 - C6 (that's 2 octaves)
Mezzo-Soprano: A3 - A5
Contralto: F3 - F5
Tenor: C3 - C5
Baritone: F2 - F4
Bass/Basso: E2 - E4

These days Jon is a Baritone and early on in his career probably was considered a tenor.

I'd have to go sit at my piano to tell you about Mark Slaughter and tonight I'm exhausted from helping my son's girlfriend move - sorry.