Lines are a fundamental principal in art. In drawing especially, lines make up every figure and shape in the image. There are many types of lines, such as straight lines, squiggly lines, and zig-zags.
Line weight is the thickness of a line. Thicker lines can be made by using tools with higher thicknesses, and applying more pressure. Lighter lines use less pressure and/or lighter tools. Line weight can be used to add some depth to an image.