Re, or Ra, was the sun god. In the Old Kingdom (the time when the pyramids were built), he was king of the gods, and as Atum-Re he was the god who created the world and human beings.
Later he was often shown with a falcon's head and the sun on his head with a snake wrapped round it. In this form he is connected to Horus, and known as Re-Horakhty (Ra, Horus of the Horizon).
Re was worshipped all over Egypt, but his main centre was Iunu, later called Heliopolis by the Greeks (Heliopolis means 'sun city').
Pharaohs always had Re as part of their throne name - for example Tutankhamun's throne name was Nebkheperure, and Hatshepsut's throne name was Maatkare.
Re was believed to sail across the sky in a boat, then when the sun went down he sailed through the underworld, facing many dangers along the way such as the giant snake Apep.
Re sailing through the underworld.