Technically, serums are water-based treatments—but you can also find some that are more like a face oil. Both are great, as long you keep in mind what results you're after.
General rule: apply after cleansing, before moisturizing, for a hit of concentrated ingredients direct to your skin.
Now, here's what they do.
1. They fight pollution.
Serums deliver a hit of intense moisture, but they also help clean out and close your pores, keeping out all the nasties. This BLITHE one is a beauty editor favorite and a twofer—it works as a moisturizer as well.
2. They fix breakouts.
At a certain point, acne solutions for teenagers just don't cut it any more—they're designed for younger (not to mention oiler) skin. But a serum does the trick while hydrating your skin to prevent dryness and general wear and tear. Just make sure you go for an oil-free one.
3. They combat aging.
Serums contain something called an "epidermal growth factor" (EGF)—this one's derived from barley—to speed up skin cell renewal and basically kickstart your skin into acting/feeling/looking younger. Put this one on overnight, and it does all the heavy lifting while you sleep.
4. They're ultra potent.
Serums on the more face-oily end of the spectrum are packed with delicious ingredients to feed your skin. This cult favorite from Vintner's Daughter is full of essential and botanical oils—22 of them—that all do different, excellent things from soothing to elasticizing. And they smell great.
5. They're like a facial for lazy people.
Serums are well-known (and very well-loved) for brightening your skin. Another very great side effect is the way they improve the effectiveness of all your other products.
Think of serum as your general face-improver in a bottle. Like a little, portable skin genie.