with Processing, to draw a circle you use ellipse function giving parameters like positions, width and height and "magically" each pixels inside this circle are colored with the fill value

with GLSL: you have 2 shaders, the first one compute positions of everything you want to draw and the other compute colors of these things.

 You could do a circle in 2 ways:

- drawing circle with vertex shader: you could send a circle as vertices to the vertex shader and the fill everything (i.e the circle) with the fragment shader

- drawing circle with fragment shader: you send a rect(0,0,width,height) to the vertex shader (so you have a surface that can be drawn on by the fragment shader), then in the fragment shader you define the position of your circle from (-1,-1) to (1,1) and a radius. The fragment shader compute color for each pixels, you can check the distance between the center of your circle and the position of the current pixel being calculated, if the distance is less than your radius it is in the circle and you can set the color you want else set another color.

Tried to make it as clear as short. Hope it helps.
Makio135
http://makio135.com/