Your eyes and your brain can play tricks on you. The mind's eye can be easily tricked into imagining lines or images where they don't really exist. Hence, we have optical illusions (also known as visual illusions) that are characterized by visually perceived images that are deceptive or misleading like the one on the left, which can be either two faces or a candle holder depending on how you choose to visualize it.
This Amoeblog includes various optical illusions for you to take a look at. You can observe the altered images as the information is gathered by your eye and processed by your brain to give you a perception that does not tally with a physical measurement of the stimulus source. You follow?
Anyways -- according to the experts in these matters, there are basically two main types of illusions. There are the physiological illusions that are the effects on the eyes and brain of excessive stimulation of a specific type -- brightness, tilt, color, or movement. And then there are cognitive illusions, where the eye and brain make unconscious inferences. That is why you may see something in an image that the person standing right next to you doesn't comprehend equally.
The optical illusion immediately above, which is one of the most commonly known, is a same color illusion in which Square A is exactly the same shade of grey as square B. The image shows what appears to be a black and white checker-board with a green cylinder resting on it that casts a shadow diagonally across the middle of the board. The black and white squares are actually different shades of gray. The image has been constructed so that "white" squares in the shadow, one of which is labeled "B," are actually the exact same gray value as "black" squares outside the shadow, one of which is labeled "A." The two squares A and B appear very different as a result of the illusion.