I've seen a lot of articles about Faeries online, but it seemed that quite a lot of them were more about Winx/Disney-type fairies then the Faeries I think of.
So here's just some of the 'darker' stuff on Faeries...:)
A good thing to keep in mind is that Faeries have different morals than we do. That does not mean they are evil; they just view right and wrong differently than we do.
For that reason, it is good to be cautious around Faeries, and to never anger them. Don't do anything that will let you see them without their permission, for example, putting Faery Ointment on your eyelids or touching your eyelids with a four-leaf clover. If you do that, any Faery you see will ask you "Which eye do you see me out of?" When you respond with either left, right, or both, they with promptly jab out whichever or both eyes, depending on which you see them with.
Another way to not anger the Faeries is to never thank them. The Faeries see this to mean that you do not mean to follow up on your debt. Most Faeries will excuse you if you thank them, as it comes automatically to humans, but some will not, and you never know what you'll get with Faery vengeance. It could be nothing more than getting your clocks all set to different times; it could be having your house burnt down. A good way to thank the Faeries, though, if you are in debt, is to give them cream, bread, honey, or milk and blood. (Careful with the cream, though--it makes them drunk!)
The Faeries don't seem to get too angry if you use a summon spell to bring them to you. After all, they will only really come if they want to. However, this will put you in debt to them, and be careful! There are some Faeries who will not help humans. Keep a protective circle around you when summoning a Faery.
Speaking of protection, if you are ever in any way contacting a Faery, it is a good idea to have protective objects around you. Iron, as most people know, burns Faeries, but it can also kill them, and the Faeries will most likely be quite angry with you. They cannot cross a line of salt, so speaking to them across that might be a good idea, as long as they agreed to it. Many Faeries cannot cross running water, but that is not true with some. Undines, for example, would have absolutely no problem crossing a river or stream.
Most people never see a Faery in their lifetime. Those who do most likely did one of two things; saw the Faeries without their permission, or have the Faeries blessing and consent.
If you are the latter, you will likely be invited to the Twilight Realm, the realm of the Faeries. It is probably alright if you do, but take caution. Some Faeries may try to enslave you, in which case you would be trapped in the Twilight Realm for a year and a day. Also, keep in mind never to eat Faery food. It is so delicious that you will not be able to stand to eat anything else. You would starve to death, unless you somehow got your hands on more Faery food.
If you ever do see a Faery or visit the Twilight Realm, take care to never tell anyone without the Faeries' permission! You will lose their blessing, and all the gifts they gave you (for Faeries give the blessed many gifts) will disappear.
Besides that, the blessing of a Faery is a wonderful thing to have.