This Video explains the concept of Curve. A curve is a line which is not straight. Simple Curve in Mathematics is a curve which does not cross itself, while a Closed Curve is a curve which crosses itself.

Refer the link below for wikipedia explanation on Curve.

http://en.wikipedia.org/wiki/Curve

The entire Chapter on Basic Geometry can be accessed at following link : -

http://iedubook.com/tutorial/ch/2/CBSE,-ICSE/CBSE-Class-VI,-ICSE-Class-VI/Mathematics/Basic-Geometrical-Ideas.html