The herdsmen likely were Jews who were raising swine for the Gentile population. According to Jewish law, the Jews were not to eat pork, nor were they to gain money by raising and selling pigs. It was Satan, not the Lord, who was responsible. Also, the owners were conducting an illegal business, so their guilt kept them from condemning the Lord. There are a number of steep banks on this portion of the Galilee, and once the swine started down, they couldn't stop.
Demons are invisible spiritual beings, part of the ranks of evil angels.
They differ in character. The ones in this episode are unclean, causing the man to have unclean thoughts and speech. They have limited and temporary power, being subject to Christ. One dysfunctional, demon-possessed man was more important than a thousand animals. Look at the man in this account.