God created the world and everything in it, and it was good. But then, Adam and Eve chose not to love and trust God. Their sin separated them from God. And that sin separates us from God too. God wanted to save His people, so he sent His Son to rescue us. Jesus came and died on the cross, taking our punishment and making a way for us to be in relationship with God. 

But, we know that everyone died. What happens then? Where is God? Romans 6:23 tells us:

For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.

Death is the actual punishment for sin - not just a physical death, but a spiritual death that keeps us separated from God forever. We will all experience a physical death, but if we accept God's free gift of Jesus and choose to follow God and let Him be our God, then we will have eternal life. We will get to be with God forever. This is where Heaven and Hell come in.

Heaven = being with God forever and having a relationship with Him.

Hell = being separated from a relationship with God forever. Hell is the opposite of Heaven, so the opposite of all the wonderful things in Heaven.

One day, God will restore His perfect creation. God will make everything perfect again. We don't know exactly what this restored world will look like, but we know that if we become a follower of Jesus, we will be there with God.