Bars are crafting materials generally crafted from mined ores at a Furnace. Crafting most ore-related items requires that raw ores are first crafted into bars.
The Calamity Mod currently adds 11 different bar types. Obtaining many of these only requires their respective ores as crafting ingredients. Astral, Auric Tesla, and Victide Bars are exceptions that require secondary materials in addition. Frigid Bars and Cosmilite Bars are not crafted but are instead dropped by Cryogen and The Devourer of Gods respectively. Some bars may also be obtained from crates after a certain boss is defeated.