Next to the ankh symbol, the icon commonly called the Eye of Horus is the next most well known. It consists of a stylized eye and eyebrow. Two lines extend from the bottom of the eye, possibly to mimic the facial markings on a falcon local to Egypt, as Horus's symbol was a falcon.
In fact, three different names are applied to this symbol: the eye of Horus, the eye of Ra, and the Wadjet. These names are based on the meaning behind the symbol, not specifically its construction. Without any context, it is impossible to definitively determine which symbol is meant.
The Eye of Horus:
Horus is the son of Osiris and nephew to Set. After Set murdered Osiris, Horus and his mother Isis set to work putting the dismembered Osiris back together and reviving him as lord of the underworld. According to one story, Horus sacrificed one of his own eyes for Osiris. In another story, Horus loses his eye in a subsequent battle with Set. As such, the symbol is connected with healing and restoration.
The symbol is also one of protection and was commonly used in protective amulets worn by both the living and the dead.
The Eye of Horus commonly, but not always. sports a blue iris. The Eye of Horus is the most common use of the eye symbol.
The Eye of Ra:
The Eye of Ra has anthropomorphic qualities and is sometimes also called the daughter of Ra. Ra sends out his eye to seek information as well as hand out wrath and vengeance against those who have insulted him. Thus, it is a much more aggressive symbol that the Eye of Horus.
The Eye is also given to a variety of goddesses such as Sekhmet, Wadjet and Bast. Sekhmet once ranged down such ferocity against a disrespectful humanity that Ra eventually had to step in to stop her from exterminating the entire race.
The Eye of Ra commonly sports a red iris.
As if that wasn't complicated enough, the concept of the Eye of Ra is often represented by another symbol entirely, a cobra wrapped around a sun-disk, often hovering over a deity's head: most often Ra. The cobra is a symbol of the goddess Wadjet, who has her own connections to the Eye symbol.
Wadjet is a cobra goddess and the patron of lower Eygpt. Depictions of Ra commonly sport a sun disk over his head and a cobra wrapped around the disk. That cobra is Wadjet, a protective deity. An Eye shown in association with a cobra is usually Wadjet, although sometimes it is an Eye of Ra.
Just to be further confusing, the Eye of Horus is sometimes called a Wadjat eye.
Pairs of Eyes:
A pair of eyes can be found on the side of some coffins. The usual interpretation is that they provide sight for the deceased since their souls live for eternity.
Orientation of Eyes:
While various sources attempt to ascribe meaning to whether a left or right eye is depicted, no rule can be applied universally. Eye symbols associated with Horus can be found in both left and right forms, for example.
People today ascribe a number of meanings to the Eye of Horus, including protection, wisdom, and revelation. It is often associated with the Eye of Providence found on US $1 bills and in Freemasonry iconography. However, it is problematic to compare these symbols' meanings beyond viewers being under the watchful eye of a superior power.
The eye of Horus is used by some occultists, including Thelemites, who consider 1904 the start of the Age of Horus. The eye is often depicted within a triangle, which might be interpreted as a symbol of elemental fire or might harken back to the Eye of Providence and other similar symbols.
Conspiracy theorists often see the Eye of Horus, the Eye of Providence, and other eye symbols as all ultimately being the same symbol. This symbol is that of the shadowy Illuminati organization which some believe to be the real power behind many governments today. As such, these eye symbols represent subjugation, control of knowledge, illusion, manipulation and power.