The drive line is a term used to describe the drive shaft and the CV shafts. This may still be a misunderstood for you, so I will give you more data to help you understand. Drive shaft and CV shafts are the links between the final drive unit or transmission to the wheel. These shafts must be flexible in order to move with suspension and steering movement. The drive shaft uses a component called a U-Joint and the CV Shaft uses 3 large ball bearings to create this pivot point. The U-Joint has four caps with needle bearings in them. The caps are directly across from each other similar to a + design. Each shaft has one of these U-Joints or ball bearings on each end of it.
When a drive shaft U-Joint starts going bad it will cause vibration that can be felt through the seat or floor of the vehicle. As the component gets worse the vibration increases, usually over time. The main cause of U-Joint deterioration is contamination violating the seal of the cap and the lack of lubrication. The CV Shaft deterioration is caused by the same contaminates, except the boot that covers the roller bearings becomes compromised by tearing or spiting open. This opening allows the lubrication to exit and contaminates to enter. The CV Shaft can cause vibration usually felt through the steering wheel. The vibration is not a fine vibration it's more like a wobble type vibration. The other noticeable sign of a CV Shaft going bad is a knocking sound on hard turns either left or right.
These components can break if the U-Joints or ball bearings are not serviced in a timely manner. When they break the vehicle will not move. Any indication of vibration, knocking, or clicking is a indicator that it's time to see the Car Doctor.