A cylindrical lens focuses light into a line, as opposed to a spherical lens that focuses light into a single point. The curved face or faces of a cylindrical lens are sections of a cylinder, and focus the image passing through it into a line parallel to the intersection of the surface of the lens and a plane tangent to it. The lens compresses the image in the direction perpendicular to this line, and leaves it unaltered in the direction parallel to it (in the tangent plane).
Applications where light needs to be focused into a line, or the aspect ration of an image needs to be changed utilize cylindrical lenses. The single cylindrical surface causes incoming light to be focused in a single dimension, ultimately stretching the image. Cylinder Lenses come with positive or negative focal lengths, ideal for laser line generation or anamorphic beam shaping to circularize laser outputs.