The Kingdom of God:

The Kingdom of God is the whole territory under the rulership and dominion of God (Yehovah).
(Meaning the whole Universe, and all that's in it.)
Also, commonly reffered to as "The Kingdom of Heaven".

What is The Kingdom of God?
The Kingdom of God refers essentially, to "The good universe" and Eternal life.
It is commonly accepted (at least amongst christians) That god is the source of all the good in the universe (something I have witnessed personally), and that "Heaven", is "Heaven", because God, is there:

There is this Parable, told by Yeshua Hamasciach ,(Commonly known as Jesus Christ) that states: "The kingdom of heaven is like a mustard seed, that a man took, and planted in his garden; and although it amongst the smallest of all seeds, when it is fully grown, it is the greatest of the herbs, and becomes a tree, so that all the birds of the sky come and rest in its branches".

There is another parable about the kingdom heaven, where Yeshua says:
"The Kingdom of Heaven, is likened unto Leaven (Yeast), which a woman took, and hid in three measures of flour, until the whole thing was risen. "
This is a parable of the world, referencing transformation, and life.
A life without God, rests upon physics, and ends in death: a life with God, is a life indeed.