The void is the black area surround the map. The map ends at this area, no blocks can be placed here, and any players going into void will be killed. If you accidentally go too high up into the map with conjur, and your gun goes above the void, you will not be able shoot anything since the void is solid; to get back to shooting, conjure blocks in the open air around you to push yourself back into the map (if stuck) or just walk left/right until you are low enough to be able to fire once again.
The barrier enclosing the void is made up of a double layer, with an approximately 1 or 2 pixel gap in between layers. On rare occasion, it is possible to get stuck in this double layer by clipping into it.
It is possible to go inside the void through glitching, but glitching above the map while immediately kill you, and if you glitch to the sides of the maps, you are not able to walk back into the map and will fall to your death. As such, glitching into the void is not useful. It is still possible to void and live, although there is no way to return except through death if a player becomes stuck below the map.
Before Version 1.24 on 6 August 2014, player used to be able to go inside the void a little ways (known as voiding), with enough room to walk above the map (but not jump) and drop down the sides of the map. It was also possible on certain maps to travel in the void underneath the map, although this wasn't very well known.