Notice how it is organized and how I've colored things. I've labeled the 3 Limb Leads yellow, the 3 Augmented Leads red, the 6 Precordial Leads blue. At the very bottom is the green Rhythm Strip (Notice in this example it is Lead I again, but it can be any lead, usually I, II or V5).
What a lead does is detect the sum total electrical potential (in millivolts, mV) of the heart pointed at a certain direction. The Limb Leads (I, II, III) and the Augmented Leads (aVL, aVR, aVF) detect the electrical potential of the heart in the Coronal Plane (It gets the “X” and “Y” axis if you thought of it like a graph):
The red thing in the center is the heart, in black are the vectors showing the direction (notice the arrowheads) the leads measure (names are in purple). Also in black is the poor outline of a person. In green are the polar degrees of each vector (more on this later when covering Axis). For example, lead I measures the direction of the electrical potential at 0 degrees, which is pointed directly to the left. Lead aVF measures the electrical potential directed directly down (or 90 degrees).