Folds
A fold occurs when one or a stack of originally flat rocks are bent or curved as a result of deformation.

As stress is applied horizontally on the layers of rocks, the rocks begin to bend. When the rock bends up, an anticline is formed. When the rock bends down, a syncline is formed.

Occasionally, the stress applied to the rocks isn't strong enough to fracture the rocks, but bend the anticline further than the apex and an overturned fold is formed.
Image courtesy Alaska Earthquake Center.