A curve is a line that moves away from being straight over time and bends at some point or along its whole length. The curves can be put into two categories, which are: Simple and non-simple curves: A non-simple curve is one that crosses itself once or twice, and a simple curve is one that doesn’t. Curves can be open or closed. A closed curve doesn’t have any endpoints and completely surrounds an area. If a curve has endpoints, it’s probably an open curve.