You probably know what a line is, but just incase you don't, it is a continuous mark made on some surface by a moving point.
There are mainly two types of lines, emphasizing lines and de-emphasizing lines.
Emphasizing lines are better known as contour lines. They show and outline the edges or contours of an object. Contour lines separate objects from one another. When artists stresses contours or outlines in their work, the piece are usually described as linear.
Not all artists emphasize line in their work. Some even try to hide the outline of objects in their pictures. The term painterly is used when describing works that does not stresses the contours or outlines of objects.