Each of the four acts in Diablo II, and the fifth act added in Diablo II: Lord of Destruction, ends with a boss battle. Your character must defeat these bosses to advance to the next act or difficulty level. Act Bosses are the most challenging monsters in the game (aside from the patch-added Uber Monsters), and also the most rewarding, in terms of the loot they drop.
Click the linked names to see a page devoted to each boss, with info about their stats, skills, and abilities.