What's the difference between a Toner, Essence, Serum and Ampoule?

Toner - Following double cleansing, toner is the third step in the K-beauty routine.  Its main function is to complete the cleansing of your skin—removing the dirt, dust,  and impurities that can still be lingering after washing with a cleanser. 

Essence - A concentrated formula to help skin cell turnover rate.

Serum - A more concentrated in its active ingredients. Helps target and treat skin issues like dark spots, wrinkles and dullness.  Usually, serums are thought to have a more viscous texture then essences.

Ampoule - A concentrated version of a serum, like a booster for your skin.