This is the promise that He Himself made to us: eternal Life. (1 John 2:25)
Everyone who has ever lived, even the unborn, have an eternal existence and an eternal consciousness. Because of their unforgiven sins, the vast majority of people are separated from God and will spend eternity separated from Him in hell or what the bible calls the lake of fire. This is eternal death; "Depart from me accused ones, into eternal fire which has been prepared for the devil and his angels." (Matthew 25:41)
A few of us, who's sins have been forgiven and have been saved by God's grace through faith in the gospel of Jesus Christ, have received eternal life now and will spend eternity with Him in heaven. This is eternal life; He who believes in the Son has eternal life. (John 3:36)
I think it is impossible for us to fully comprehend the concept of eternity. Eternal life is not just about unending life. Our eternal life transcends and is independent of time. The New Testament Greek word for "eternal" is aionios, which carries the idea of quality of life, as well as quantity of life.
Our physical life is short and, like a cloud, quickly passes away; "Vanity of vanities", says the preacher.(Ecclesiastes1:2). But even though this life is short, our lives have significance and lasting consequences for eternity; So then each one of us will give an account of himself to God. (Romans 14:12) As a Christian, you have eternal life. Now, in light of your eternal life, live every day to the glory of God!