The Tel'aran'rhiod (commonly called "Dreamworld" or "Dream realm") is the mysterious plane connected to Azeroth and Emerald Dream. It is said that it is mirror to waking world. Only few know about its existence and even fewer about its purpose. The only beings if they can be called such that exist there are Nightmares. Also it is possible to get hurt or even die if visitors are not careful.
Entering the Dreamworld is usually done while sleeping. Most people by mistake enter it for few seconds wile the very few has the ability to enter and remain. The time spent there is usually not same as the time spent sleeping. It is also possible to physically enter the dreamworld but it is rumored that by doing so, the person looses part of his being. If entered physically it is possible to travel from place you entered to new location but as already mentioned this rarely advised.
Corruption appears in many forms and its sources are possibly the Nightmares. Most common corruption is pure darkens but surroundings usually change drastically fast and in unpredictable ways. The source of this are thought to be Old Gods or their minions Nightmares. Recently this was changed by Calliope and Leviathan which turned the world pure white. According to Prophet that is only short phase of cleansing.
Manifestations of fear, they are darkness that comes to dreams of all mortals without them even knowing. There is relatively not much knowledge about them considering they never were encountered knowingly by most races. In their realm they hunt for unwary travelers and usually kill them. They have been seen working for Old Gods but were they created or recruited its unknown. What is known is that not all Nightmares serve Old Gods from which most notable is Shadow but their motives aren't necessarily good either. They also have shown the ability to leave their world but how is unknown. They have also been know to enter Emerald Dream.
Only few people can be listed as such. Most notable are Leviathan, Mur'ar, Calliope, Alagremm and Trolman (who was dragged there to his trial which was moved for unknown future). Many of Leviathan's High Priests are familiar with dreamworld and use it for emergencies. Also Green Dragons are possibly visitors duo to connection between two realms.
The nature of Dreamworld is so powerful that it can literally express persons thoughts and hidden desires. Usually the changes wear off but if it is case powerful creature as Titans changes can be of very long effect. It also possible to interact physically but effect also fade off duo to nature of world to mimic waking world.
-Tel'aran'rhiod is an actual Dreamworld from Robert Jordan series "wheel of time"
-The world was first in DP intentioned to be drowned wasteland created by Leviathan sadness. Explanation for this now is unique nature of world to express persons thoughts.