Different types of halos

One of the more common attributes seen in religious artwork is the halo, also known as the nimbus. These circles of light around a figure's head are usually thought to be a symbol of divine origin or a holy nature, but it turns out that it's a bit more complex than that.

Halos are a visual shorthand to some quality of the figure they surround. For example, certain designs are restricted to members of the Trinity, while another design separates living people from other Saints in the same picture. Probably the biggest example of halos meaning something other than holiness comes from Eastern art, where they have been used to show power instead of holiness or virtue. This has resulted in art that depicts figures like the Beast or even Satan himself adorned with a halo.

Below is a short overview of some types of halos and what they might be used to represent. When a tutorial is available, the name of the design will be clickable.
Basic Halo
The simplest of all halo designs is just a ring around the figure's head. This is also the basis for most of the other halo designs, so if you're having trouble drawing halos, this would be the one to practice.
Cruciform Halo
Sometimes referred to as a Triradiant Halo because of the three rays shown within it, this is a special design that can be worn by Jesus Christ. The crosslike rays are a reference to His sacrifical death on the Cross at Calvary.
Cruciform Halo (alternate)
The Cruciform Halo can also be drawn with some additional embellishments, such as a monogram or having the rays extend beyond the boundries of the halo's circle.
Elevated Halo
This is yet another variant of the Cruciform Halo. In this instance, the entire halo is raised up, or elevated, higher than normal, drawing more attention to the three rays inside the halo.
Greek Hexagonal
One of the more elaborate halo designs, this one is created by combining two triangles. However, unlike the Triangular Halo above, this halo is reserved for figures that represent God the Father.
Nimbus a Pans
Probably one of the more unusual halos you'll ever come across, this design is used to denote allegorical figures, such as a personification of Justice or a Virtue.
Ray Halo
Another type of halo used by figures representing a member of the Trinity is the Ray Halo. This halo is meant to resemble rays of light spreading out in all directions from the person wearing it.

Occasionally, rays like this will be added behind other halos worn by the Trinity, such as the Triangular Halo.
Square Halo
Sometimes a scene will depict both heavenly Saints and living people. You could simply leave the head of the living person without a halo, but some artists choose to use a square halo instead.

There's also a precedent for using squares like this; see the page on shapes for more about using a square to represent Earthly things.
Starred Halo
Shown only on the Virgin Mary, this decorated halo appears to be a reference to Revelation 12:1, which describes the mother of Jesus Christ as a woman clothed by the sun, standing on the moon, and wearing a garland of stars.
Triangular Halo
Triangles are often used as a simple way to represent the Trinity, and so it's not very surprising that artists would use a halo in the shape of a triangle to show that a figure represents a member of the Trinity.