Welcome to Humor Nation. They are dangerous, they are corrupted, they are DEMONS! We first came across the demons in Season 1 of the Supernatural and they proved to be just too much for the Winchester. A demon is actually born out of a human soul that has been through extreme punishment and suffering, this torture takes away their punishment and innocence, and makes them evil and corrupted. A demon requires a meat suit AKA human vessel to walk free on the earth similar to the angels. However, they can escape in their smoke form. Over the course of 15 seasons, Demons have created plenty of problems for the Winchester. However, there have been some demons who proved to be formidable allies for the Winchesters.
There are numerous types of demons. The most common kind is a black-eyes demon which are the most ordinary type, however they possess super strength. However, one can weaken them using the devil trap, salt, and holy water. All black-eyed Demons are not same, some of them are more powerful and skilled than the other such as Meg and Ruby. Then comes the Crossroad Demons who can be easily identified with their signature red eyes, they serve the King of Hell and are tasked with making deals with the desperate and the needy. They make deals of all sorts in exchange for the souls which the people have to give after a certain period of time like 10 years. In case of Dean, he was given only a year when he asked the crossroad demon to bring back Sam from the dead.
Then comes the White Eyed Demons who are extremely powerful to the extent that they can hold their ground against the angels. They are just below the Lucifer. So far, the only known demons are Lilith and Alastair. Then comes the Yellow eyed ones who are the princes of hell who were created after Lilith and belong to the first generation of demons. Azazel, the leader of the demon army who planned to free Lucifer. Other demons of these kind includes Ramiel, Dagon, and Asmodeus.