Demons are very real. My dad went to Africa on a mission trip and saw a woman who was delivered from evil spirits. She was screaming and had convulsions and tried to bite his leg. It was pretty scary the way he described it.
Yes, they are angels who turned against God when Satan tried to take over. They will try to kill, steal, and destroy you any way they can including taking possession of you. They can be removed by faith in Jesus and praying. However, it helps to have more than just one person with you when you pray because there is strength in numbers. Also be sure you and the others are born again followers of Christ, or they will come back again with seven more even more powerful demons to take hold of that person again.
To believe in Jesus Christ is to also believe in demons. Since he openly cast them out of people that acknowledged His capability to do so. He was also confronted by demons when he was tempted in the desert. The confusion comes when there are so many different definition of what a demon is from a ghost to a being that totally takes over a persons body. At http://www.aredemonsreal.net you will find more information about demons and how they impact the life of individuals.