A banshee (in the Irish language, bean sidhe) is usually considered part faerie, part ghost.
However, with the banshee, the line between those labels blurs.
A banshee may represent an actual ancestor, so she’s like a ghost. She may be a shapeshifter as well, so she’s more mystical or fae.
Many people think a banshee causes death. That’s absolutely false. The banshee protects the family she’s attached to.
Other people think the banshee predicts death. That’s not always the case.
A banshee warns family members about a potential, upcoming tragedy.
Sometimes, that tragedy can be avoided. At other times, it cannot. The banshee wails because she sees the tragedy and, on her own, she can’t stop it. However, another family member — someone living in physical form, at that moment — might be able to.
Most families with Irish ancestry have their own, individual banshees. They may never know she’s protecting them. (Scottish families, and those with Scottish ancestry, are more likely to have a green lady. She protects the house and those in it.)
Recently, some people have speculated that each banshee was profoundly psychic during her lifetime. So, there may be “banshees in training” that are very much alive right now. In fact, that makes sense, since the network of banshees expands with each generation.
Banshees probably aren’t trapped in our world. Not unless the banshee’s unfinished business includes guarding her descendants. I believe she manifests in this plane by choice, and she helps the family she tries to protect.
Also, her protection doesn’t usually extend to harming others, even enemies of her family. She will warn and defend her family, but she is not an aggressor.
If you study Irish folklore and history, you’ll understand banshees far better.