Christians have often interpreted these verses in light of the expected return of Jesus. The faithful will be rewarded; the sinful will be punished. Eventually, the new Jerusalem is seen as heaven (as it is described in Revelation); the valley of Jehoshaphat is seen as a euphemism for hell.
[links to Bible Gateway open in a new window]Reading the Bible in Its Entirety
If you are planning to read the Bible in its entirety, you should read Joel 2:30-3:8 today.