To the ancient Egyptians, Ra was the ruler of the heavens. He was the god of the sun, the bringer of light, and patron to the pharaohs. According to legend, the sun travels the skies as Ra drives his chariot through the heavens. Although he originally was associated only with the midday sun, as time went by, Ra became connected to the sun's presence all day long. He was the commander of not only the sky, but the earth and the underworld as well.
Ra is nearly always portrayed with a solar disc above his head, and often takes on the aspect of a falcon. Ra is different from most Egyptian gods. Other than Osiris, nearly all deities of Egypt are tied to the earth. Ra, however, is strictly a celestial god. It is from his position in the sky that he is able to watch over his independent (and often unruly) children. On earth, Horus rules as Ra's proxy.
For people in ancient Egypt, the sun was a source of life. It was power and energy, light and warmth. It was what made the crops grow each season, so it is no surprise that the cult of Ra had immense power and was widespread. By the time of around the fourth dynasty, the pharaohs themselves were seen as incarnations of Ra, thus giving them absolute power. Many a king build a temple or pyramid in his honor - after all, keeping Ra happy virtually guaranteed a long and prosperous reign as pharaoh.
When the Roman Empire embraced Christianity, the residents of Egypt rather abruptly abandoned their old gods, and the cult of Ra vanished into the history books. Today, there are some Egyptian reconstructionists, or followers of Kemeticism, who still honor Ra as the supreme god of the sun.