1. Anticlines: These are folds that arch upward, like the top of a hill. The oldest rocks are found in the core of the anticline, with younger rocks on the flanks.

2. Synclines: These folds curve downward, like a valley. The youngest rocks are found in the core of the syncline, with older rocks on the flanks.

3. Monoclines: These are folds that create a step-like structure, where the rock layers are tilted on one side and horizontal on the other.

It's important to note that these are just the three main types, and there are many variations within each category. For example, a fold can be symmetrical or asymmetrical, open or tight, and even overturned.