Instances are areas which zone player(s) into a special area. These unique areas have NPCs which do not respawn after they die. Instances may have entry requirements of level or number or quests. Some have wait times until they can be re-entered.
To access information in game, hit F10, and select <Instance> from the list.
1 = solo = blue 2 = arena = orange 3 = alliance = lightgreen 4 = abyss = lilac