An example of an optical illusion is two people facing each other, nose to nose. The faces themselves are black, and the space between them is white. If you look hard enough, you will see a vase between the vases, comprised of the white space. Some people see the vase first, and upon further gazing, see the two people facing each other.
An example of an optical illusion is an image created to be paradoxical in nature, such as a staircase drawn to appear that it is going up or down.