I posted a blog about knocking in my friends walls.I guess most people believe it could be a ghost... I don't know anything about them at all... are they all happy and nice? Can they hurt you at all.

Ghosts are people who are dead and don't know or realize it or people who have died tragically or suddenly and don't realize it. Sometimes evil entities actually trap good entities in areas... and yes, they can be rather mean if they want to... the evil entities that is. It's quite complicated and simple all at the same time. They can't really "hurt" you but they can make your life miserable.

There are the poltergist ghosts but they tend to be mean right from the start. I have never heard of a playful, mischievous ghost switching good/bad.

There is really no right or wrong answer here. Yeah ghosts can be mischievous and playful and yes they can be downright nasty. I've experienced both. When I was little I used tohear my name being called and things would disappear and reappear in the weirdest places. My remote ended up in the refrigerator? I've also been pushed and even scratched. But you can usually tell right away what you're dealing with.