What is Heaven? The basic meaning of the word translated "heaven" in Hebrew, shamayim, means "skies" or "the heights." Similarly, in Greek, ouranos is basically "sky" or "air." In both Hebrew and Greek, then, the word refers to the sky above the earth. In post-exilic Judaism, "Heaven" became a way to refer to God. So, "Kingdom of Heaven" is the Kingdom of God. "I have sinned against Heaven" means I have sinned against God. Heaven also became the word used to refer to God's habitation, which is what we will focus on here.
Heaven is the place where God dwells. He "sits" in the Heavens (Ps. 2:4); He "looks" down from Heaven upon everyone (Ps. 14:2; 33:13; Is. 63:15). Heaven is God's "holy" place (Ps. 102:19). God's throne is in Heaven (P. 103:19). In fact, Heaven IS God's throne (Is. 66:1; Matt. 5:34). Sometimes the Lord is called "The God of Heaven" (Gen. 24:7; Ezra 1:2; Dan. 2:37).
Heaven, then, is that spiritual place where God dwells. The Bible tells us some surprising things about that spiritual place. It is not a place of golden harps, little angels with wings, clouds, white robes, etc.
Here are some pictures of the "current" Heaven. "Heaven" is the site of struggle and of spiritual warfare. The Bible tells us that there was war in Heaven with Satan (Rev. 12:7-12), and Satan was cast out. There was no longer any place for him in Heaven. The Bible tells us that there are spiritual forces of wickedness in the "heavenly places” (Eph 6:12). Those who have died and have left this earth are presently "unclothed” (2 Cor. 5:1-10), and have not received their new, spiritual bodies (1 Cor. 15:44). Is that Heaven? In Job 1:6, the accuser comes into God's presence, presumably in Heaven (Job 1:6). The souls of the martyrs are pictured in turmoil under the altar (Rev. 6:9). They are told to "rest" awhile longer (Rev. 6:11). Is this the eternal rest? No, it is not. This is not the Heaven that is in store for us eternally.
The Heavens and the earth were created together, and together they will be destroyed (Is. 51:6; 2 Pet. 3:7-10). The Bible tells us that the Heavens and the earth will be "changed" (Heb 1:10-12). The Heavens and the Earth will be remade into a new Heaven and Earth (Is. 65:17; 66:22). This present creation will be "redeemed" from corruption (Rom. 8:21). God will make a "New Heaven and a New Earth" (Rev 21:1). This New Heaven and New Earth is what we are looking for (2 Pet. 3:13).
So, Heaven is that spiritual realm where God dwells, as well as other spiritual beings. There apparently are spiritual forces of wickedness in this realm at the present time. However, God will destroy it all and make a new realm for us where righteousness dwells. This final place is what Jesus has gone to prepare for us (John 14:1-3). That preparation is not complete just yet, but it will be.