In a series of nice video visualizations, you can see how your browser draws and orders the different elements of a web page. There are examples for Wikipedia, Google, and Mozilla.org . Click here to view.
From Mozilla: "Reflow is the process by which the geometry of the layout engine's formatting objects are computed. The HTML formatting objects are called frames: a frame corresponds to the geometric information for (roughly) a single element in the content model; the frames are arranged into a hierarchy that parallels the containment hierarchy in the content model. A frame is rectangular, with width, height, and an offset from the parent frame that contains it."