What's the difference between a serum and a cream?
Short answer
The books don't define the terms side by side, but their formulas show the difference: the serum is a water-based gel, while the creams are emulsions of oil and water.
What the books don't cover: The books don't define the two terms side by side.
Explanation
The Copper Peptide Renewal Serum is built on a light gel of water, hyaluronic acid and glycerin thickened with Aristoflex AVC, carrying peptides and needing no oil phase. The Barrier Repair Cream and Cellular Renewal Elixir are oil-in-water emulsions: an oil phase with an emulsifier, heated and combined with a water phase. The book's oil-only products, such as the Omega-Renewal Facial Elixir, are a third type, with no water at all.
Source: The Big Book of Homemade Skin Care — Copper Peptide Renewal Serum · Barrier Repair Cream