Orbs are spheres, balls of light in the sky or close to the ground, whose origin and composition is a mystery. That is the definition off the top of my head for my purposes here. They are probably the most photographed paranormal phenomenon to date.
Most of them cannot be seen with the naked eye, but I have seen some of them in the bright light of the flash, and I have seen one fly into my house and go through the wall while my cat and I both watched in awe. I have a friend in Phoenix who saw several of them come into her house once, and I did see several of them, orange ones, flying around a hill behind the Post Office in Sedona one morning around 7 a.m.    

# 10

Over Mt. Adams, Washington State. Taken at James Gilliland's Sativa Sanctuary, August, 2002.

# 9

In movement. Slow shutter speed.

# 8

My previous home in Sedona and orb over chimney
in my painting studio. Scientists say the curvature of the camera lens may curve and bend the light of the orb around the edges of the photo, but the chimney here is not affected by the curvature of the lens, so why is the orb? Because it is a light reflection, and not a 3D object?

# 7

In this photo, my friend Tami is shaking a blanket while I am photographing with my Sony digital. The camera has captured the dust and it looks like orbs. Note the appearance of them, transparent and faint. These are different from the opaque, luminous orbs in other photos I have taken. I don't believe all orbs are the same phenomena. Differentiating them is an issue with orb study.

# 6

Note some in the trees at lower left. This is an early orb photo of mine that hooked me on chasing them to learn about them, dust or otherwise!

# 5

Enlargement. Some are unusually bright. Golden orbs have been associated with etheric bodies or souls in the literature. See Swanson's book, The Synchronized Universe.

# 4

Taken in Cornville, AZ. There may be more than one kind of orb here---the bright one could be an earth light, a UFO, or an astral body.

# 3

An average "orb night" in Sedona. Take your own photos at twilight, a good time to catch them.

# 2 There are different kinds of orbs. Some are transparent, some are transparent with concentric circles in them, some are opaque and self-luminous, some are textured, or organic-looking with holes in them. Some appear to have faces. They can be bent, like a concave jellyfish, or look like someone took a bite out of them. I do not believe they are all the same in composition or origin. Some have responded to my thoughts, and thoughts of my friends,and thoughts of crop-circle researchers. Can a dust orb do that?
These blue orbs appeared in my photo when I asked for colorful ones. Blue is my favorite color; how did they know? Was I just projecting my own thought onto them through the camera? I have more questions than answers.