An electromagnetic wave
Light and all other forms of electromagnetic radiation can be thought of as waves of electromagnetic force passing through space. This diagram is, in a sense, a graph showing how the intensity of the electrical and magnetic fields associated with a light beam vary at different points in the beam. From a value of zero each field increases to a maximum, then decreases to zero again, then increases to a maximum in the opposite direction, with the electrical and magnetic fields always being at right angles to each other.
The distance between each maximum and the next maximum in the same direction is called the wavelength of the wave associated with the light beam. The intensity of the field is a measure of the brightness of the light beam (or more accurately, of that particular wavelength of light in the beam). Real light beams would be thought of as a large number of waves with different intensities, wavelengths, and positioning relative to each other, all moving through space in the same direction and at the same rate -- the speed of light.
According to the laws of electromagnetism the light beam once created moves through space by altering the conditions within the space that it is passing through. The changing magnetic field generates a changing electric field which is identical to the electric field of the beam and the changing electric field generates a changing magnetic field which is identical to the magnetic field of the beam; so that once the beam begins moving in a particular direction the changing electromagnetic field creates a continually regenerating field identical to the original light beam. The beam is said to be propagating through space rather than merely moving through it, because of the way in which it is, in a sense, recreating itself from moment to moment as it moves through space-time.