The difficulty of quests can vary significantly. The following terms are used to give an idea of what you should expect when attempting a specific quest.
In your Quest Journal, on the right, It should say Heroic or Epic. If it says nothing, then the quest is Solo.
Solo[]
This quest is designed to be completable by a single player of the level indicated. Some steps may require the help of a friend or 2.
Heroic[]
This quest is designed for a full group of 6 players within the level range indicated. There may be earlier steps that are soloable, but the final step will require friends.
Epic[]
This quest is designed for 2-4 full groups (12-24 players) within the level range indicated. There may be earlier steps that are soloable or can be done by a single group, but the final step will require a lot of friends.
Generally speaking, Epic Quests are some of the most difficult and challenging content in the game, and should not be attempted lightly.