Dumbbells are free weights with equal mass on both ends of a short handle, so the load sits in line with your hand. That balanced design makes them easy to control for classic strength moves such as presses, rows, curls, and lunges, and it suits both one-arm and two-arm work.
Kettlebells place most of the mass below a single handle. The offset center of mass favors swinging, ballistic, and two-handed moves (for example swings and goblet squats) and often adds a larger demand for power, core stability, and conditioning in the same session.
Neither tool is universally better for building muscle: both can develop strength when load and technique are progressed. Choose dumbbells for balanced, controlled lifting; choose kettlebells when you want more explosive, whole-body training—or use both. Source: Dumbbells vs Kettlebells — ISSA.