Line art is simply an image consisting of lines, straight or curved, without any gradation of shade or color. Areas of solid pigment and dots can also be used in addition to lines. Line art can use lines of different colors, although line art is usually monochromatic.
Vector Line Graphic is often used to describe images that contain lines or areas of solid color with clear and distinctive separation between each color- no color gradients or blends from one color to another but a distinct separation of color.
LEFT: Original Photograph RIGHT: Vector line art
Technically, this isn't Line Art, not the way we see it, but vector graphics can be "seen" by certain software as simple lines, so I am making up this category to differentiate from continuous tone images made with pixels or images that include gradients which cannot be represented properly by lines. More on this.
Below is a sample of a photograph vectorized as a vector illustration. The photograph on the left has many shades of color often called continuous tone. The vector line illustrations on the right has a distinct separation between each color.
Vector line graphics are required for processes that are guided by the vector outline such as engravers and certain types of signage.