One level of attachment is an earthbound spirit or ghost. Some of these may not be ready to move on or may have a message for someone, or seeking closure.
Thought forms are another attachment and are not from a spirit. This can be from negative comments years ago that you have latched on to.
Negative energy is yet another type and this can be from a spirit or a negative event.
Psychic attack or as I call them psychic vampires are people who can mentally or psychically suck the happiness right out of you.
Curses, cords, and vows are just a few more that all kind of lump together. If you believe in a thing such as a curse than of course it can affect you too. A cord is really a mental thing attaching you to something else and a vow can be almost like a promise or even a curse depending on what was said.
Past life attachments (what many have said I am currently dealing with) is when someone from a past life attaches itself to your current self. They could want closure or forgiveness for something in your past life.
There are many more types of attachments people can deal with, all depending on your belief system. From extraterrestrials, demons, elements, etc. A lot of these all have to do with what you believe in and if you welcome them in or are open to them. Not always though, I have never welcomed anything in from the paranormal field but when you open yourself up by being involved in the field, you never know what may happen or what you may come across.
Author/Founder of Historic Haunts Investigations