Three constructions dominate the market. Innerspring mattresses use a coil support core, usually topped with a foam or fiber comfort layer, and tend to sleep cooler and feel more traditionally bouncy. All-foam mattresses use layered foam densities instead of coils, contour more closely to the body, and are the standard "bed-in-a-box" build. Hybrids combine a coil core with a thicker foam or latex comfort layer on top, aiming for the support of springs with more of the contouring feel of foam — and they're now the most common mid-to-upper-tier option in stores.
Firmness is usually rated on a rough 1–10 soft-to-firm scale, but the number means little without context: a 6 from one manufacturer can feel like a 7 from another, since there's no industry standard behind the scale. Side sleepers generally want more give at the shoulder and hip (softer to medium); back and stomach sleepers generally want more support (medium to firm) — but body weight changes this more than sleep position alone, and a good salesperson will ask your weight before your position.
In-store mattress trial periods commonly run 90 to 365 nights, but almost all of them exclude the first two to four weeks as a mandatory break-in period before a return is accepted — ask for that number specifically, since it's easy to miss. A boxed mattress needs roughly 24 to 72 hours to fully expand after unboxing, though most are usable well before that. Delivery for mattresses is usually threshold or room-of-choice rather than white glove, since there's little to assemble; ask separately whether old-mattress haul-away is included or a paid add-on.
A store that pushes you toward the model on promotion rather than asking about your sleep position, weight and current mattress complaints is optimizing for its own inventory, not your sleep.