Folds are bends in rocks that are due to compressional forces. Folds are most visible in rocks that layered (also known as sedimentary rocks). Folds are formed when heat and pressure is applied to the rock. The higher the temperature, the more pliable rocks become. Folds are more likely to occur when the deformation caused by the compression is applied slowly.
If the pressure (compression) that is applied to a rock undergoing a fold is greater than the internal strength of the rock, then the rock will fracture. This is how faults are formed. Faults are defined as the displacement of rock that were once connected along a fault line.