There could be a number of reasons for that, however in general this phenomenon manifests the poor condition of one or more elements of suspension. If vibrations occur during driving at steady speed or during idle running, they may point to bad wheels balance. If this is not the case, another reason may be a deformed or damaged wheel or rim. If vibration is felt during speeding up, the reason is most probably damage to the power transmission system. The first thing to be checked in such case is the condition of drive joints. If vibrations occur when braking, the most probable reason are bent brake back plates. This is easily identifiable, since vibrations increase during braking at higher speed.