The Linear projection is also known as a planar projection. The texture projects based on a single direction, and is best for very simple shapes. It's useful when mapping textures onto planes that act as cards or matte paintings for distant scenery, or for images projected on a flat screen. Figure 1 shows a render of a sphere that uses a Linear projection to map a Checker texture to the surface. You can connect a Transform node to the Transform pin, which allows you to control the texture’s surface position.
Figure 1: A Linear projection of a Checker texture onto a sphere