A Boss is a monster that is more powerful than a normal monster. It is mostly surrounded by it's minions. :Bosses often reside on the other side of large, ornate doors, which frequently lock once the Hero enters the room.
Most Bosses are from quests, but are also good for raids or farming. Most Bosses drop Green (magical) to Purple (legendary) items, but it can vary. Low level bosses mostly don't drop more than yellow.
Bosses are often the guardians of Majestic Chests. Majestic Chests contain the highest chance of obtaining a rare or powerful item, and are thus highly valued by players.
A boss can also be called 'quest monster' or 'leader'. A boss usually have a kind of 'aura' around them and are rarely seen if not from a quest.