This representation of an electro-magnetic wave uses vectors to show the strength and direction of the electric and magnetic fields along the x-axis as the wave propagates to the right (positive x-axis direction) at the speed of light.  The length of each vector "arrow" indicates the strength of the field and the direction in which the "arrow" points indicates the direction or orientation of the field.  Note that the electric and magnetic field directions are always perpendicular to each other and to the direction of propagation of the wave, hence an E&M wave is a transverse wave.  Click on the diagram to see a Java applet of an E&M wave in motion.  Unlike a mechanical wave, an E&M wave does not propagate through a medium.  An accelerating charge creates a disturbance in the electric and magnetic fields which surround the charge which propagates away from the charge at the speed of light in a vacuum (c = 3.0 x 10⁸ m/s).  The speed of light in a vacuum is a universal constant and the basic wave equation for E&M waves would be written:  c = lf