If you are are into skincare you would eventually run into these terms. Popular brands such as Estée Lauder, SK-II, Shiseido, Murad, Caudalie and AmorePacific tend to carry these products in their skincare collection. Essence, serum and emulsion are generally promoted in a 5-6 skincare regimen. This is a trendy practice in Asian countries and it is gradually going global. Before you stop by Sephora or your local department store and pick up one of these, learn the difference and see if you actually need it in your skincare routine.
This product is generally applied after cleansing and toning your face. Technically, it is a type of moisturizer that does more than hydrating your skin. Serum usually contains a few key active ingredients to address specific issues like brightening, dark spots, wrinkles, etc. In addition, the active ingredient is more concentrated than a regular moisturizing cream. The thicker is also thicker (like a mix of oil and lotion).
Here is tricky one, the term essences and serums are practically interchangeable. They actually do the same thing but the only difference is essence is more lightweight and less concentrated than serums. Hence, it is traditionally used after toner as another layer of hydration before the serum. If you prefer a lighter and more watery texture go with essence!
Think of this product as a light moisturizer. It is usually water-base and it's used as an alternative to moisturizer (which is thicker in consistency). People with oily or combination skin tend to gravitate toward emulsion because it hydrates skin without giving a sticky or greasy residue. Most emulsion are water-based instead of oil and cream.