Costumes are different visual skins that are available for your heroes. They can be equip through the seamstress in the Wardrobe tab upon achieving LVL __ on at least 1 character.
Costumes are acquired mainly through premium currency and (along with bag space, character slots and premium pets) the main way Trendy hopes to make money through DD2 which is a Free To Play game. As such in order to avoid a pay-to-win scenario, costumes are only cosmetic, affecting animations and graphics but not providing stat benefits.
Costumes are unlocked through DLC or through purchase from the seamstress. Costumes are categorized into sets, each which contains at least 1 costume for:
Head Armor Torso Legs
The costume can be constructed with parts from only 1 set but you cannot currently mix parts between sets.
Some costume piece can be acquired through Lockboxes which can be opened either with gems or Wyvern Tokens (through the purchase of Keys)