Dumbbells are small bars with added weights at both ends that are normally used with one hand. Depending on the type of dumbbell these weights are fixed or interchangeable. They are used for weight lifting or strength endurance exercises.
Handle lengths of dumbbells vary, but are around 30-40cm (11-16"). Weights are fixed to the bar or changeable, starting from a few hundred grams up to ones physical limits.
Questions are about everything that is related to dumbbells, eg equipment questions or exercise recommendations.
Related tags are barbell for the long dual handed type of bar or kettlebell for a type of weight with a ring shaped handle.