An out of shape tyre is a distortion normally in the treaded area cause by failure of the cords (belt ply), the cords that run under the treaded area is part of what give the tyre its strength and to keep the treaded area flat.
The image below shows the tyre tread on the left is normal and the tyre tread on the right has lost is structural shape (out of shape) Believe it or not this is the same tyre just turned 180 degrees.
An out of shape tyre would have a high spot in the tread area and cause vibration through the vehicle, if you feel the vibration more through the steering wheel then its most likely to be one of the front tyres out of shape, if its more of a car shake then it will more likely be one of the rear tyres.
Its not uncommon for a vehicle to develop a pull either left of right if one of the front tyres has gone out of shape.
If you experience vibration at speeds under 40mph to should immediately have your vehicle checked as this could be a tyre that is out of shape which could fail while Driving causing life threatening danger to you and other motorists.