Gears exist to keep your pedalling effort roughly consistent whether you're climbing, on the flat, or picking up speed on a descent. The right setup depends far more on your terrain and riding style than on having "more" gears.
A derailleur system — the familiar chain-and-cage mechanism moving the chain across a cluster of rear sprockets — is what most mountain, hybrid, and geared roadster bikes use. It's light, efficient, and gives a wide spread of gears for varied terrain, but the exposed mechanism needs occasional cleaning and adjustment, and it can't be shifted while stationary.
A hub gear system moves the gearing inside the rear wheel hub itself. It's sealed against dust and rain, can be shifted while stopped at a signal, and needs far less maintenance — a real advantage for a daily commuter bike ridden in all weather. The trade-off is a narrower gear range and slightly more weight, which is why it's less common on bikes built for hill climbing.
Single-speed bikes have no gear mechanism at all — one fixed ratio, nothing to adjust or maintain. They make sense for flat commutes and kids' first bikes, where simplicity and low maintenance matter more than climbing ease.
Gear count on a spec sheet ("21-speed", "7-speed") is the number of front chainrings multiplied by rear sprockets, not a measure of quality. A well-maintained 7-speed hub gear can outlast a neglected 21-speed derailleur — what matters is matching the setup to how and where you actually ride.