Although Jenna's answer is essentially correct, the underlying cause of such sensations, at least as it relates to passengers, is the body's inability to define where it is in space. By that I mean, in an airplane, unlike, say an amusement ride, there is no outside, fixed point of reference. As an example, when on a boat or ship one will often hear the advice to fix your sight on the horizon. This does help quite afew people afflicted with sea sickness. However, in an aircraft, the balance sensory mechanism of the inner ear is telling the brain one thing and the eyes are telling it another, with no fixed-in-space point of reference other than the aircraft itself, which is of no help, since it is moving as well. The attempt to reconcile these often opposing inputs can cause the uneasy feeling sometimes leading to air sickness. (This is one reason pilots easily overcome such sensations since they have the luxury of looking out the windscreen at the horizon or relying on instruments that relay the same information). To fully answer your question, the sensations are most often caused by turbulence, but can be caused by the aircraft banking in a turn or the beginning of a descent or climb... anything that causes inputs into the semi-circular canals of the inner ear that need to be resolved...