Often it's like driving along a bumpy road, but sometimes it is much more violent. It sometimes seems associated with passing through sections of cloud but at other times it occurs in clear sky and it seems to happen more at high altitudes. There are several reasons why you might get a bumpy ride while flying in apparently clear air. The air in the atmosphere is constantly changing and moving to a greater or lesser degree, so things are rarely still even if there is no cloud.
Clouds are visible forms of water vapour which occur when saturated air condenses.
The actual nature of the cloud and its size depends on numerous factors including the moisture content and stability of the air. Aircraft motion can be affected by clouds but, as we can see below, clouds are only the most easily noticeable of the many atmospheric factors. All aeroplanes produce lift through their wings in order to fly. Generally, the bigger the aircraft the greater the turbulence in its wake. This wake can continue for some distance behind the aircraft, and while you may feel nothing in the aircraft in which you are travelling, passengers in another aircraft passing through the same altitude may fly through your wake and feel some turbulence.