Heaven is everything the universe was supposed to be. People think fluffy white clouds and harps, but heaven (meaning the paradise after this universe is destroyed in blazing fire and melting of the elements after the 1,000 year reign of Christ on earth) is actually the new heavens and earth. There will be peace and perfect unity between all people, there will be no killing or death of anything, and there will be time (infinite) to explore the universe and learn from and about God (who is endless).
Hell, by contrast, is existence (eternal) outside God's kingdom. The physical reality of it is utter darkness, and the spiritual reality is one contemplating sin for eternity. Hell is reserved for those who want no part of God's kingdom here on earth.