A halo is a thin ring of light that sometimes appears around the sun or the moon. Halos are formed by the refraction (bending) of light as it passes through small, hexagonal (six-sided), pencil-shaped ice crystals high in the sky.
The two most common types of halo are the 22 degrees halo and the 46 degrees halo. The number of degrees in a halo refers to the angle by which light is refracted through the ice crystals. For instance, if light is bent at an angle of 22 degrees, it will form a circle of light with a radius (the length of a line segment from the center of a circle to its boundary) of 22 degrees. The 22-degree halo is smaller than the 46-degree halo and more tightly encircles the sun.
A 22-degree halo is formed by ice crystals that are randomly arranged. The light enters one of the six sides and exits through another of the six sides. The ice crystals that produce a 46-degrees halo are all arranged the same way, so that sunlight strikes one of a crystal's six sides and exits through one of its two ends.
Haloes may form at the leading edge of an advancing storm system. For that reason, they are often looked upon as a sign of rain. However, a halo is not an entirely reliable forecasting tool; the storm front may change direction or gently pass through without producing rain.
Sources: Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the ment, 5th ed., pp. 98-99; Engelbert, Phillis. The Complete Weather Resource, vol. 2, pp. 328-29.