Zones in BO3 pretty much work the same way they did in World at War, so this page will only cover the changes in BO3. For a full guide to zones, refer to Ultimate Guide to Zones in v1.4 Maps.
Drag a info\volume from the entity browser into your map and give the volume the zone KVPs you learned from the WaW guide.
Zones now also act as playable area for players. If you want a zone to denote an area that the player is allowed to be, add the following KVP to it:
You'll notice that if you don't have any player volumes in your map, you'll die as soon as you spawn into your map.