Area boundary

These are often referred to as "portals".

On the world map, area boundaries are marked as small white/bluish vortexes.

These mark where you can move from one zone to another. An area boundary is represented by a glimmering curtain of bluish-white light (or, in the Nightfall Campaign, a thick ring of golden light) through which you can see the adjacent area. Walking through this curtain results in the next zone being loaded.

The following mechanics exist with regards to passing through a portal -
 * If one party member passes through the area boundary, all members of the party will be transported with him, regardless of where they were. This can be useful to reunite lost or dead party members (the dead will be resurrected), although going back and forth between areas is usually not the preferred method of resurrection. When you re-enter an explorable area, all mobs the party encountered will also be resurrected, and reset to their initial positions.
 * Passing through a portal while one or more party members are not loaded (gray health bar in the party window) will kick them from the party.
 * If heroes or henchmen pass through a portal, they will be teleported to the location of a human party member, and any flags that sent them there will be removed.

Most area boundaries have a Resurrection Shrine nearby; many of those that don't will act as resurrection shrines.