When cell membranes in the heart depolarise, voltages change and currents flow.
Because a human can be regarded as a bag of salt water (with baad attitude), in other words,
a volume conductor, changes in potential are transmitted throughout the body, and
can be measured. When the heart depolarises, it's convenient (and fairly accurate) to
represent the electrical activity as a dipole --- a vector between two point charges.
Remember that a vector has both a size (magnitude), and a direction.
By looking at how the potential varies around the volume conductor, one can get an idea
of the direction of the vector. This applies to all intra-cardiac events, so we can
talk about a vector (or axis) for P waves, the QRS complex, T waves, and so on.
Read the whole tutorial at anaesthetist.com
Bibliography and sources
A good general reference is Leo Schamroth's An Introduction to Electrocardiography, published by Blackwell Scientific. (7th Ed., 1990,
ISBN 0-632-02411-9). It has the merits of both clarity, accuracy and depth). Leo
was one of the truly great men of electrocardiography, and a brilliant physician,