I'll try to summarize my thoughts on the subject:
1. No one really and truly knows a lot about the subject. Only small fragments of information can be gleaned from a mass of speculation. I'll issue the caveat that I'm no expert, don't know much, and could be wrong on one or more points. I've tried very hard to sift through information that ranges from obviously nutty to unprovable. I've found that a very limited number of people seem to have real info on the subject. They all are willing to admit that they don't understand much of what is going on. I myself had the aforementioned encounter in the 90's and have also been involved in an ongoing situation in recent years. (I don't really want to give a lot of detail on the ongoing situation. However, I've drawn a LOT of conclusions from it.) I've talked with a handful of otherwise rational, intelligent, professional, and level headed people that seem to have legitimate experiences. I'm not talking about the kind of people you see on TV, but people that don't even really like to talk about it openly. It is VERY difficult to get people to talk about the real deal.
2. I really wish that I could still think of demons as a metaphor for human problems and struggles instead of a real possibility. Most people's demons are symbolic. However, not all are purely symbolic. Some are very real. I've even come to the conclusion that they are fairly common.
3. Think of demons as predators. Just like a predator they can hunt in packs, look for the weak and those separated from the "herd", etc.
4. Mental illness can be very commonly mistaken for demonic troubles. You NEVER want to tell a mentally ill person that they have problem with demons. It WILL make their problems worse. That being said, the mentally ill are often attacked by demons. It goes back to the fact that demons are predatory and often look for the "weak" to "hunt". The same goes for many other issues and problems that people have such as abuse, drugs, etc. They often seek out those that are easy targets.
5. We think of possession as a demon "living" inside of a person. I'm not 100% sure that is the actual case. I have a theory that they are not actually physically located anywhere in particular in the strict sense of physical terms. Think of a chess game. You aren't "inside" of the game or the individual pieces when you play. You are observing the game and reaching in and manipulating the pieces on the board. I don't know if that is the best analogy but understand that I'm not sure that they are limited to a single location at any particular moment.
6. They are varied in type, power, interests, etc. They aren't carbon copies of each other. We often classify animals in the biological world. You can think of demons as a "class" of entities as well.
7. They are very evil, persistent, vengeful, capable of team work, immortal, intelligent, and you really don't want to mess with them useless you are forced into the situation.
8. There is some sort of connection between demons and some ghost phenomenon. I'm not saying that all ghost phenomenon is caused by demons but at least some percentage of it is related to demonic activity. They like to mimic dead people and are good at it.
9. They tend not to operate openly. They like to be the hidden influence. They especially don't like to become known to people that don't believe in them.
10. They have at least some ability to read minds, know the past, and predict the future. This makes them very dangerous.
11. Demons are all about deception and lies.
12. Do not try to communicate with demons. They tend to follow and attack people that communicate with them.
13. Do not try to communicate with any spirit or ghost. Demons like to mimic dead people. This is the real reason that ouija boards and similar forms of "communication" are dangerous. They regard talking to them as "permission" to attack you.
14. Exorcism isn't an instant cure. It is an ongoing battle with attacks and counter attacks until they move on to easier targets. In the second situation that I alluded to, we performed multiple house blessings (ie., minor exorcisms) before it had a major effect.
15. Demons may "haunt" locations, people, and even families through generations.
16. You can't get rid of demons if you keep letting them back into your life. No amount of exorcism will work if you keep allowing them back in.
17. For whatever reason, some people are more susceptible to demonic influence than others. I spoke with a minister that has very quietly dealt with demonic situations for a long time and he told me that he had seen cases when a child had become very involved in satanism for a time and never appeared to come under demonic attack and other cases when a child did something only a single time and was attacked from all directions.
18. Demons appear to attack women at a much higher rate than they attack men. That isn't to say that they don't attack men. However, they seem to attack women at a very much greater rate. I don't have a good theory as to why.
19. A lot of people dismiss the topic of demons/exorcism because it is both very, very scary and has strong world view/religious implications. Most people prefer a head in the sand approach to the issue.
20. Demons are inhuman. By that I mean that they have never been human. They are intelligent non-human, non-biological entities. (Some people would argue that some demons might be former humans. I don't know about that one way or the other.)
21. As to the origin of demons, Christian theology generally points to the idea that they are non-human intelligent beings (ie., angels) that served God at one time but have rebelled and now war against both God and mankind. I believe this to be the case and my belief is supported by the fact that demons have a very strong aversion to the name of Christ, invocation of the authority of God, etc. My wife and I have observed this in person with the ongoing situation that I referred to earlier. As a matter of fact, after one of the last house blessings that we did in that situation, a disembodied voice spoke to my wife when she was alone and said, "You may have gotten rid of us but wait until you meet our father." I'm not joking or making that up. My wife is an intelligent, knowledgeable, and educated person. At one time, she didn't even believe in the existence of demons.
22. There are several levels of demonic influence in the following order from least to worst: infestation, oppression, and possession. Infestation and oppression are by far the most common types of demonic influence. Infestation normally takes the form of generic haunting type activities and is the precursor to an oppression attempt. (Infestation is more indirect in nature.) Oppression is a gradually increasing series of attacks that impose thoughts, feelings, emotions, etc. that seek to isolate and mentally weaken a person. Possession occurs whenever the degree of demonic influence is so strong that they can directly and quickly control a person. I don't know how common possession is. However, I think that infestation and oppression are actually very common.
23. Demonic activity cannot be measured in an empirical fashion. Therefore no one can "prove" anything about it. It is a topic that is full of peril from both an intellectual, personal, and theological standpoint. Furthermore, you run the risk of sounding like a nut if you talk about it. Lastly, it doesn't help that a lot of crazies are attracted to the subject and see demons around every corner and under every rock.
24. Harry Potter, Dungeons and Dragons, etc. are not demonic. They are simple fiction and fun. Too much time is wasted on red herrings and it makes logical discussion of the subject difficult. Some of you will think that the fact that I regard ouija boards as a problem is contradictory to the statement that I just made. I'll admit that is confusing but too many people have had very negative results from ouija boards and other attempts at communication. I think therein lies the difference. I think it is the act of communication that makes the difference. If you take a person with a natural venerability and combine that with even a half-hearted attempt at communication then you could have a problem.
25. Demons are attracted to strife, anger, abuse, hate, etc. They thrive in that type of environment. If you or someone you are with is having demonic trouble, you must get control of your emotions, problems, temptations, etc. or your own issues will be used as a weapon against you and people around you.
26. Demons look for something to give them permission or a foothold to enter in and then expand their control and influence. An analogy might be a dyke with a leak. At first the water comes through slowly but as the hole erodes out, more and more water begins to come through until the dyke breaks and a wall of water comes in.
27. They are capable of imposing temptations, thoughts, etc. on anyone. Don't think that you are immune. The danger lies in acting on those initial thoughts and emotions. Their influence grows as you act on those thoughts. By the way, I'm not saying that all temptations, bad thoughts, etc. are demonic. (Most are not.) I'm saying that if demons are attacking someone or a group of people, they will use imposed thoughts/emotions as weapons against you.
28. I've studied cases where various methods are used to "exorcise" demons from locations or people. Most of the time, a "Hollywood" type exorcism is not done. Simple prayers, house blessings, and help of the person to overcome their issues are highly effective in many cases. Sitting someone in a chair and doing an exorcism on them is a last resort for hardcore cases.
29. Christian prayers, symbols, etc. are more effective than other methods. I've studied a number of other peoples cases and in my own observations have found that to be true.
30. I'll repeat that I could be wrong on some points. However, once you have personally seen demonic activity. It will change your world view.