To my knowledge the word serum is like the word milk, it refers more to consistency. Serums are light and most of what they do is cosmetic, like gel, and don't moisturize or otherwise contribute to bettering your hair. Most mainstream serums are -cone based, but there other serums made up of different ingredients, usually light oils.
3C/Fine to Medium/Low Porosity/High Density
My hair is having a bromance with glycerin!
CJ Gentle Cleansing Shampoo
DB Pumpkin Seed Curl Moisturizing Conditioner
CJ Curl Assurance Smoothing Lotion
Styler: CJ Curls in a Bottle
No Salts, No Waxes, No Butters, No Heavy Oils
No ACV, No Saponified Oils, Protein Sensitive