"Serums are thin-viscosity topical products that contain concentrated amounts of active ingredients," Dr. Tsippora Shainhouse, board-certified dermatologist at SkinSafe Dermatology and Skin Care, told Healthline. The publication went on to explain that there are large active molecules in serums that are able to penetrate the skin, so that skin is able to receive optimal results.

Eyelash serums are similar to skin serums in the sense that biotin, castor oil, and hyaluronic acids are usually key ingredients — and all of these have been attributed to hair growth (via Self). Something to keep in mind with eyelash serums is while they can help to accelerate lash development, you don't want to get the product into your eyes, so they need to be applied with extreme care — and with an applicator. 

Also, while there are plenty of eyelash serums on the market, it should go on record that the only one that's FDA-approved is the brand Latisse. So, if you're looking for something that is a sure thing, that is the bottle that you should pick up first. Consistent application of it could have you boldly batting a beautiful set of eyelashes in no time!