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.
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.
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.
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.