Okay, so we know that an equation of (x-a)^2 + (y-b)^2 = R^2 is the equation for a circle. Basically where x-a is change in the x-axis and y-b is the change in the y-axis. Here is the thing that I don't get, as quoted from my book. This equation in R^3 (three dimension) defines the cylinder of radius R whose central axis is the vertical line through (a,b,0). How does it define a cylinder?