Boss (or skull) monsters are among the most difficult in the game, and often provide strong loot upon defeating them. There are also some unique mechanics to boss-monsters that normal mobs don't have or use.
See boss encounters
Boss-level monsters are treated as having 15 higher weapon skill than the player's cap. This reduces all of the player's avoidance abilities by .6%; for example, a player with a dodge chance of 20% only has a 19.4% dodge chance against a boss. Additionally, the boss is treated as having a considerably higher set of avoidances than you would expect. A player's base miss rate against a boss increases by 4% (9% for two or one-handed attacks, 28% for dual-wielding), the boss's dodge rate increases by 1.25% to 6.25%, and their parry rate greatly increases to ~11% (although the parry rate can vary from boss to boss).
Bosses have a much higher chance than normal mobs to resist incoming spells. Instead of the base 5% chance to resist incoming spells, bosses resist 17% of all incoming spells. This resistance can be reduces to 1%, although all mobs retain a 1% chance to resist incoming spells, via Hit Rating.
Crushing Blows are no longer in the game with Wrath of the Lich King.
Boss-level monsters have a 15% chance to deal 150% damage on a normal attack against a defense-capped player. While for some classes (like druids) this is unavoidable, both warriors and paladins can stack enough block chance to prevent crushing blows while their buffs are active.
Glancing Blows are no longer in the game with Wrath of the Lich King.
Players have a 25% chance to deal 70% damage to a boss-level monster with melee autoattacks and a capped weapon skill. This does not effect hunters's autoshot ability.