The zone keyword on a power indicates that it creates a lingering effect over an area. This lingering effect, called a zone, generally fills the same area of effect as the power that created it.[PH:59]
Zones generally can't be attacked, and can't be affected by attacks or environmental effects.
If a zone's creator is allowed to move the zone, however, at least one square of the zone must remain within the range of the power that created the zone. If the zone's creator dies or moves out of range of the zone, the zone's effects immediately end.
A creature within multiple overlapping zones is affected by all of them, and takes damage from each zone that deals damage.[U:5/2010] Other penalties do not stack, however, so the creature is only affected by the largest one of each type of penalty.
The May 2010 update allowed zone damage to stack. Previously, a creature affected by multiple zones only took damage from the most damaging one. Under that rule, zones were not achieving their intended damage output.