Modes

There are 3 main modes in Terraria RPG, as well as 8 submodes. Each have many bosses, weapons and item.

Normal Mode
Normal Mode is the first mode of the game. It lasts from Level 1 - Level 455.

Normal Pre-Hardmode
Normal Pre-Hardmode features bosses from proper Terraria's Pre-Hardmode, excluding King Slime and the Brain of Cthulhu. It lasts from Level 1 - Level 55. In total, the bosses (in order) are:
 * Eater of Worlds
 * Eye of Cthulhu
 * Queen Bee
 * Skeletron
 * Wall of Flesh

Normal Hardmode
Normal Hardmode features bosses from proper Terraria's Hardmode, excluding Queen Slime and Empress of Light. It lasts from Level 55 - Level 140. In total, the bosses (in order) are:
 * Skeletron Prime
 * The Twins
 * Destroyer
 * Plantera
 * Golem
 * Duke Fishron

Normal Lunarmode
Normal Lunarmode features bosses from the Celestial Events and the Moon Lord, excluding the Lunatic Cultist. It lasts from Level 140 - Level 200. In total, the bosses (in order) are:
 * Vortex Pillar
 * Stardust Pillar
 * Nebula Pillar
 * Solar Pillar
 * Moon Lord

Normal Calamitymode
Normal Calamitymode features bosses from the Calamity Mod, from Providence to Supreme Calamitas, but also including several minibosses such as the Eidolon Worm and the removed LORDE boss. Note that while some of the bosses in this list are minibosses in the Calamity Mod, in the Terraria RPG game they are treated as regular bosses. It lasts from Level 200 - Level 455, and is the longest submode in Normal Mode. In total, the bosses (in order) are:
 * Profaned Guardians
 * Providence
 * Signus, Envoy of the Devourer
 * Ceaseless Void
 * Storm Weaver
 * Polterghast
 * Colossal Squid
 * Reaper Shark
 * Juvenile Eidolon Worm
 * Devourer of Gods
 * Bumblebirb
 * Jungle Dragon, Yharon
 * Supreme Calamitas
 * Adult Eidolon Worm
 * The LORDE

Expert Mode
Expert Mode is the second mode of the game, and features bosses that are upgrades to their Normal Mode variants. The amount of bosses stay the same, mostly.