Any line is a curve. Line can be straight or bended
Curves are formed by drawing without lifting the pen from the paper
Simple Curve
A curve which does not cross itself is a simple curve.
Here,
(1) & (2) are simple curves
(3) & (4) are not simple curves
Closed Curve
A curve which has no open ends is a closed curve
Here,
(2) & (4) are closed curves
(1) & (3) are not closed curves