Shadertoy: You write a program that is executed once for every pixel on the screen and produces the pixel's color.

This: You write a program that is executed once for every vertex (the number of which you specify) and produces the vertex's position and color. Vertices are then drawn to the screen using the primitive type you specify: With points, each vertex results in a point (a small square, really); with triangles, three vertexes result in a triangle and so on.