What is a past life?
A past life is a life you had before this one, where you were another being. Some people believe that the soul is not bond to one body, and once the body dies the soul moves to another body ether on this plane or another, sometimes its many years later, sometimes only a few.
How do you know if you have one?
Theres a lot of ways to find out. Meditation is probably the simplest. I think most of us do, since this world is so old (probably much, much older than we all think) but I think its possible some of us are new souls on their first time in life.
Are they good or bad?
They're neither, if you ask me. Its like saying life is good or bad. Theres good parts and theres bad parts.
How do you release them?
Learn about them first, find out what the bad parts where and cleanse yourself of the karma that may have been passed over. Then try not to make the same mistakes you made in that life.
What is a past life regression?
Its having memories of your past life/lives or knowing who you were or what happened. It can be induced and it can come naturally as well.