Bosses

Bosses are large, aggressive, resilient enemies that offer a substantial challenge to players. Defeating bosses is usually instrumental in advancing the game in some way. Each has its own particular way of being summoned, and warning messages appear whenever one is about to appear. Defeating a boss concludes with a congratulatory message and several items. Most of the bosses can pass through blocks, with exception of King Slime, Lepus, and Golem.

During Hardmode, each night has a one in two chance of spawning one of the three Mechanical Bosses that have not yet been defeated in the current world.

Pre-Hardmode Bosses
Are the ones you initially start the game with before you progress to certain mile stones (mostly defeating these).

Eye of Cthulhu
http://bulbapedia.bulbagarden.net/wiki/File:448Lucario-Mega.png Click on me! : )

Eater of Worlds
The Demonite Ore Can also be sold for More than seven Gold coins a stack! This is largely considered the best boss farming boss in the game. (Pre hardmode).

Hardmode Bosses
Hardmode Bosses are significantly harder upgrades of and new additions to previous bosses. Defeating them is usually instrumental in advancing the game in some way. Each has its own particular way of being summoned, and warning messages appear whenever one is about to appear. Defeating a boss concludes with a congratulatory message and several items.

During Hardmode, each night has a one in two chance of spawning one of the three Mechanical Bosses that have not yet been defeated in the current world.