Fold is when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic deformation.
Two types of folds:
–Anticline: is a fold that is convex
–Syncline: is a fold that is concave
And in this picture we can see the differents parts of a fold
pictures: wikipedia and http://earthsci.org/education/teacher/basicgeol/deform/deform.html