The Ethereal Plane is a foggy expanse with low visibility divided into two distinct domains, the Border Ethereal and Deep Ethereal. The Border Ethereal exists as an empty parallel plane to the Material and Elemental Planes known as the Inner Planes. Here, structures and geographical features are mirrored but shrouded in a gray fog. This part of the Ethereal Plane is an empty echo of the Material Plane, overlapping but not interfering.
The Deep Ethereal lacks these orientating reference points, with only roiling mists churning in all directions. However, travelers of the Deep Ethereal will occasionally see flashes of light that can lead them to random locations within the Border Ethereal.
An adventurer might seek to travel to the Ethereal Plane for several reasons. A creature on the Ethereal Plane cannot be seen or heard by someone on the Material Plane, allowing it to be used strategically to avoid enemies. Similarly, a creature in the Border Ethereal can bypass solid objects on the Material Plane. This makes infiltrating a location without proper defenses against extraplanar incursions all too easy for creatures that can travel between the Ethereal and Material Plane.
Those brave enough to travel to the Deep Ethereal can use the Ethereal Plane as a gateway to travel the multiverse. Curtains of vaporous color are scattered throughout the Deep Ethereal that, when passed through, can lead travelers to regions of the Border Ethereal connected to one of the Elemental Planes, the Material Plane, the Feywild, or the Shadowfell. This makes the knowledge of how to safely travel the Ethereal Plane a powerful bargaining chip for purveyors of extraplanar travel.