If a creature is ethereal, it is in the Ethereal Realm. This is the domain of spirits and does not take kindly to those with a physical form. Ethereal beings cannot typically be interacted with nor sensed by material (non-ethereal) ones. The main except is that if an ethereal being passes within 5 feet of a material one, the material creature feels a strange unnatural chill pass over them.
If a creature that's ethereal is not innately a spirit, then they still have a size and shape while ethereal, but it is blurry and transparent. They cannot pass through material (non-ethereal) walls or objects. However, they also cannot push, pull, grab, nor interact with material objects in any way. It is as though the ethereal creature has no material strength whatsoever.
If you go down while ethereal, you die, become a wisp and your body dissipates. Any being that has a physical form and that isn't being kept ethereal by some effect will manifest back into the Material Realm at dawn. They remember what they experienced while ethereal, but the details and order of events is a little jumbled and hazy.