Demons are broken down into 6 classes, the first being the weakest and most common and the last being the strongest and most dangerous.
There are many different types of demons and not all are inherently bad, though many can be. Depending on the level of the demon, certain demons are actually allowed to roam earth while some escape from hell and have to be hunted down.
Demon Classes:
Level 1: The most common and weakest of all demon classes. Many of the demons in this class are actually harmless and are often allowed to roam freely through out earth as long as they don't harm humans. Some demons are even considered friends or pets of the guardians.
Level 2: The demons in this class tend to be trouble makers but often times do very little damage or harm to humans. These kinds of demons are ones who cause people to stumble, spill food on their shirts or drop their papers everywhere. Most aren't harmful but some are.
Level 3: This class of demons consist of demons who don't actively seek to hurt humans but end up doing so in the long run. A lot of these demons attach to humans at a young age and cause behavioral problems or other such issues. Demons level 3 and above are not allowed to pass through hell freely and roam earth. Many do however sneak out and escape causing amok. These demons often live in the minds of people and cannot be removed without harming the human.
Level 4: High ranking demons who are often seeking to hunt and kill humanity. These demons hold a grudge for the guardians and can be hostile towards them. They just want to be free and run amok on earth. These are demons that can form pacts with humans to escape hell.
Level 5: Only two known species exist in this level. These consist of the Time Demons and the Blood Demons. Both of these species of demons are on pare with the Guardians and are some of the only demons strong enough to kill guardians, other than another guardian. The Blood and Time demons have a hatred for one and other and are constantly at war with one and other trying to eliminate the other kind in order to become more powerful. These demons seek destruction of all over demons. There are old (like several thousand of years old) rumors of a third class of demons that are on this level known as Dimensional demons. Though no one actually has seen a dimensional demon, and most believe it is a myth spread by angles to make the guardians work more diligently.
Level S: Only consists of one demon. The highest and strongest demon exists on this plane however this demon is powerless unless it is able to be released/ escapes from hell. This demon is Satan/ Lucifer himself ("Honestly the S rank is there just to make him feel special" ~ Azriel).