Construction: foam on foam vs. foam on springs
The definitions are boring but they explain everything downstream. A memory foam mattress is an all-foam stack — polyurethane or viscoelastic foam up top, firmer support foam underneath. A hybrid keeps a foam comfort layer (usually including memory foam) but puts it on top of individually wrapped coils. Five of the six reviewers define them this way, with no real disagreement.
That single structural difference produces the rest of this article. Coils mean airflow, bounce, weight capacity, and mass. Foam means contour, silence, and heat retention.
One footnote the sources bicker over: everyone credits NASA with memory foam's origin, but they disagree on the decade and the purpose. Some say the 1960s for aircraft cushion safety, one says the 1980s, another says 1960s rocket protection. It's a fun fact, not a buying criterion.