If you're looking for a motocross or off-road tire, it's easy: Pick the surface you ride -- desert, forest, hard rock or dirt -- and a quick internet search will return your perfect tire. The biggest differences are the tread and the pressure it's designed to run at.
But now imagine you're not only going to ride the wilds. You want to ride the freeway to the hills, catch a fire road to a single-track, then ride the freeway home. So you're going to need a tire that can do both.
To help you out, many manufacturers list percentages, for example, "20% off-road, 80% street." If you can figure out how much you ride, where, it makes picking a tire easy. As you'll see, the big difference is tread -- deep, spaced lugs for off-road, and tightly packed, shallow lugs for street.
The right balance is up to you.