Regardless of whether you're box modeling or point-for-point modeling, understanding the concepts of loops and rings definitely makes your life as a modeler a lot less crazy.
Generally speaking, an edge loop is a series of edges that connect to form a path where the first and last edges connect to each other — well, that's the ideal case anyway. I like to call this kind of closed edge loop a “good” edge loop.
Of course, then you probably want to know what a “bad” edge loop is. Well, you can have a path of edges that don't connect at the beginning and end of the loop, but calling these loops bad isn't really accurate. It's better to refer to edge loops that stop before reconnecting with their beginning as terminating edge loops. While you generally want to avoid creating terminating edge loops in your models, you can't always avoid having them, and sometimes you actually need them for controlling how edges flow along the surface of your mesh.
To get a better understanding of the difference between closed edge loops and terminating edge loops, open Blender and add a UV sphere (Shift+AMeshUV Sphere) and leave the default settings for rings, segments, and radii. Tab into Edit mode on the sphere and Alt+right-click one ...