Why is Jesus called the “Last Adam”?

Adam was the first man created and he sinned, causing a separation between God and his creation.  We are all decedents of Adam and therefore all have this separation.  We are all born of Adam, have the sin nature, and are destined to die in a cursed world.  Jesus, who was fully man and fully God, came to restore the creation after Adam.  Through our Redeemer, we can be born again  and live with him in Heaven for eternity, where there will be no more curse.

The problems of the world started with something hanging from a tree, and those problems were resolved by God hanging from a tree (Source: Answers in Genesis)