A SmartNet is the most basic form of a Moto trunk system. It's a small system run by a single site-controller computer (only one control channel is ever heard on the system).
A SmartZone is an advanced system that has multiple transmitting sites linked together over a wide area. They come in 2 major different flavors:
1. A system like the NJSP uses multiple sites over each troop area to broadcast the control channel over the same frequency at every site. Most major talkgroups (patrol, medevac, DOT) are simulcast on all sites and can be heard from one end of the troop to the other (quite simple).
2. The new system in Burlington County (South Jersey) is made up of 10 different geographic zones, and each zone has it's own frequencies and it's own control channel. If, (let's say), an ambulance is on a call in the north zone, you'll pick up the transmission there if you're tuned to that zone. Now, the EMS unit is heading down to a hospital in another zone with the patient (here's where it gets complicated), the system will follow him on his original talkgroup out of the zone and in to the new one (let's say the hospital is in the north-central zone, you'll start to see the same TGID for the north zone now show up in north-central as well, simultaneously. Someone can correct me on this, but I think this kind of system is a SmartZone OmniLink.
This second kind of system is quite hard to trunk properly with the scanners that do CC-only, because the scanner is going to lock on to the first control channel it finds, not the strongest.
Sorry for the ramble! But to sum this up quick - A SmartZone was designed to cover a large area, and a SmartNet for a small area...