Heaven

Heaven is an ageless, divine realm that is the native homeworld of angels. Heaven is where most good people go after death, however many people--both good and evil, Christian and non-Christian alike--have made their way to Heaven, making the exact requirements to go to Heaven unclear. Heaven consists of infinite realms, each one corresponding to the likings, desires and memories of each person.

The center of Heaven is called The Garden (Zachariah refers to it as "Mission Control") and, like the rest of the realms, each person sees it how they want to. The Garden can be reached by the Axis Mundi, which is often seen as a river or a road, depending on the person.

Heaven is implied to be an actual physical realm in the same universe as earth. Anna states that when she fell, she did so literally--as in crash-landing from outer-space at a speed of about 10,000 miles per hour. Imagery from the sky-scape of Dean's Heaven shows a solar eclipse, placing Heaven behind the moon (a possible reason for the episode title). Also, the fact that the devil's gate covers a hole in the ground places Hell in the same physical plane as earth as well, providing symmetry to the whole idea.

Most demons fear Heaven's inhabitants, God and his angels, and will flee from them on sight. Some of the more powerful demons like Alastair can send angels back to Heaven, but only someone with an angel or archangel's sword, God, or Death, can kill an angel. Despite the fact that Uriel states that only an angel can kill another angel, Zachariah is killed by Dean. At one point God was in Heaven; this was when he sent Castiel to Hell to save Dean Winchester, but for some unknown reason, left Heaven afterwards and still hasn't returned. According to Joshua, God is walking the Earth.

Since God's abandonment of Heaven, Michael is assumed to have taken command as his right as "Viceroy of Heaven" with some assistance from his younger brother, Raphael. After being trapped in Lucifer's Cage, it is confirmed that the ruling of Heaven and the Angelic Host has fallen onto Raphael's shoulders, leading it like a Dictatorship, it is assumed that Castiel took command after killing Raphael but, since Castiel's demise, it is unknown if Heaven is ruled by another powerful Angel or if Castiel left someone in Command. It is even possible that, due to the quiet from Angels that has befallen since Castiel's supposed death, that Heaven may have slipped back into Civil War.

Trivia

 * Ash tells the brothers that Heaven is comprised of roughly 100 billion individual Heavens. Considering that the number of humans that lived and died since 50,000 BC is estimated to be ≈106.5 billion, that puts roughly 5% of the total human population in Hell. This means that, in the Supernatural Universe, religion is not a deciding factor for who goes up and who goes down.