Dumbbells are lifting weights meant to be used with one hand, most often in pair. They are composed of two equal weights, generally of circular shape, linked by a central handle. The bigger and heavier version, designed for two-hand use, is called the barbell.
They are either fixed-weight, which is simple but not always efficent for home gyms, or adjustable, with various mechanisms allowing a to change the weight.
Similarly to the kettlebell, their history seems to go back as far as Antiquity and beyond, with different cultures using different shapes for a variety of strength training purposes.
Dumbbells are especially useful to develop upper body strength and muscles.
I started with a pair of 10 kg adjustable dumbbells. Handles weight 2 kg, and come with two discs of 2 kg and four discs of 1 kg. When I became more serious about lifting, I switched to a pair of Powerblocks.