Pareidolia is a type of illusion or misperception involving a vague or obscure stimulus being perceived as something clear and distinct.
It seems to be human nature to try and form images that we know by experience from things that may not be so obvious. Shapes in clouds, "faces" in tortillas and orbs, and even the face on Mars are examples of pareidolia. There are many things that could be perceived as having human traits but is merely an "illusion".
Photographs can also yeild interesting results in pareidolia, leaving the photographer wondering if he has caught the image of a ghost in their photo.
Tree in the shape of a human.
An excellent example of pareidolia. Submitted by Dylan, this photo shows what appears to be a man with hat and glasses in the background.
It is actually a plant...