Sheen is the level of shine in a paint finish, and it changes more than looks. Higher sheens are more durable and easier to wipe down, but they reflect light and show every roller mark and wall imperfection. Lower sheens hide flaws and look softer, but they scuff and stain more easily.
Flat or matte belongs on ceilings and on walls in low-traffic rooms like adult bedrooms and formal living rooms, especially older walls that aren't perfectly smooth. Eggshell is the workhorse for most living areas, hallways, and dining rooms: a slight glow, decent washability, and forgiving on minor flaws.
Satin steps up for kitchens, bathrooms, kids' rooms, and mudrooms, where walls get wiped regularly and moisture is a factor. Semi-gloss is for trim, doors, cabinets, and window casings, where you want a hard, cleanable surface and a little contrast against the walls.
One caveat for older Metro Detroit homes: plaster and heavily patched walls look best in flat or eggshell. If you want satin in a room with imperfect walls, plan on drywall repair and skim-coating first, or the sheen will point out every bump.