We don't know what to call these things. Some people call them wraiths, others call them revenants, and others still call them spectres, apparitions, phantoms and lemures. Spanish practitioners call them fantasmas. Japanese occultists call them yūrei.
Your little kid would call them ghosts.
Long story short is this; if you die with any serious emotional baggage that could keep you from resting in peace, this is what happens to you. You don't cross over. Your spirit stays here until those matters are settled and your soul is put at ease. If your soul -isn't- put at ease, you start to change. All the good things about your personality fall apart as the emotional pain and existential turmoil wear at you from inside. It gets even worse if you're stuck here because you want revenge on someone. Any spirit who goes down that path is guaranteed to lose their shit before the end of their days.
And no, there's no peace for them when it's all over. The hateful dead don't go to Heaven.
I need you to understand right now that the ghost stories you hear at the campfire are chicken shit compared to the real thing. Real phantoms kill your favorite magicians every Tuesday night. They have powers that would give you nightmares, and you need specific kinds of magic to even touch them. Physical strikes don't work, most spells can't even get a hold on them, and even spiritual attacks don't stun them as badly as living creatures. And don't even -think- about trying to mess with their minds. You do that, and the experience will leave you crying.
You think -that's- bad? It gets worse. The longer a spirit is left to walk the Earth with whatever emotional knapsack the person was carrying when they died, the stronger and madder they become. A wraith that's been around for six months is bad. A wraith that's been around for six years is that much worse.
I'm not even going to mention what happens if a wraith reaches anything higher than a century. The moment it gets to that point, you stop trying to help it and start running in the opposite direction.