The All-Seeing Eye, sometimes called the Eye of Providence, represents God's omniscience and omnipresence and has been used by innumerable organisations and cults. It appears on the Great Seal of the United States, its dollar bills, on bills of other countries (Ukraine and Estonia), and on the tee-shirts of rock band artists.
It is used by some as an apotropaic talisman, to reflect back harm. It appears as such in Islam and Judaism with the Phoenician Hamsa (Khamsa or Chamsa), although strict Muslims and Jews consider such talismans a form of idolatry.
In Islam, the Hamsa is known as the Hand of Fatima or Eye of Fatima. Only one of Muhammad's children, Fatima, or al-Zahra (the shining one), survived into adulthood and was able to bear descendants from Muhammad's line. She has, therefore, taken on a revered position in Islamic thought, just as Mary has with Catholics.
The Jewish name for the Hamsa is Hand of Miriam, in reference to the Prophetess in Exod. 2:1-10, sister of Moses and Aaron, and inspiration for modern day Jewish feminists. (These are not your bra-burners and Germaine Greer womyn, but a group of people dedicated to seeing that women can take a more active role in the historically male-dominated religion.)
Such talismans are meant to protect the holder from the 'Evil Eye' - a curse invoked by the sin of envy, or simply looking upon something with envy.
Eyes have a profound psychological effect on us, and for this reason, an image of an eye (or pair of eyes) is used in many contexts.
An experiment in 2006 at Newcastle University, Northern England, showed that a photograph of a pair of man's eyes, looking directly at the observer, induced honesty. (http://www.newscientist.com/article.ns?id=dn9424)
Over-large eyes can be cute or scary, depending on the context (see the children's cartoon anime character on the right).
Sunglasses can look like huge dark eyes. Police and soldiers wear them to intimidate; gangsters and pimps in cocaine-white Mercedes wear them for disguise. (The Blues Brothers look cool in shades, but otherwise, wearing them at night, indoors or perched on top of the head just looks comical.)
Eyes can also be made to look larger with make-up. American footballers started wearing eye black at the top of their cheeks in the 1940s to reduce glare from the sun, and found it also had useful intimidating effects.
Like in many other countries, schoolgirls in Japan rebel against their high school uniform code at the weekend by donning extreme fashion. A new style every season of course, and excessive eye make-up is essential. (The yamamba style for example - "I've just got back from surfing in Hawaii. This tan and my panda eyes prove it.")
Staying on the topic of wild animals, we see birds, insects and fish use false eyes or ocelli to fool predators.
In the case of the 'Owl' butterfly, the large 'eye' on its wing is used both as a decoy (predators attack the wing rather than the body) and as a repellent (predators think the eye is from a larger adversary).
Many kinds of fish are attracted to lures painted with eyes.
Eyes are painted on boats to protect the superstitious fishermen from the 'Evil Eye' (fascinatio in Roman mythology, maloccio in modern Italian) - a curse invoked by the sin of envy, which can initiate a spate of bad fortune. Popularly believed in many Mediterranean cultures, one protection is to counter the Evil Eye with a pair of painted eyes, as seen on the upper bow of this Maltese fishing boat.
All of this is far removed from the All-Seeing Eye Cross; a symbol used by Christians. This symbol comprises an eye and a cross, and often includes a triangle and/or rays.
If there are rays, they often number twelve, reminding us of the twelve disciples. Rays are not meant to represent the sun, rather they are rays of Glory.
If a triangle is included, this represents the Trinity of God the Father, Jesus Christ His Son, and the Holy Spirit. (Note: a pyramid has all sorts of alleged supernatural properties, which most people dismiss as 'pyramidiotic' notions. On this cross, we show a triangle. A triangle is not a pyramid.)
The All-Seeing Eye itself symbolises the 'eye' of God, keeping watch on mankind. How we interpret that is up to each of us. We could say that God is watching every move we make, so we'd better behave ourselves. That's true, of course, but sounds a bit scary (see also God fearing). Alternatively, it can be seen as God watching over us, like a father, ready to lend a hand if we call on Him (see also Guardian Angel).