A construction permit is like a temporary license. It usually lasts for three years. During that time period, the station must get its facilites operating at the new parameters. A new station, for example, must get itself on the air. If an existing station applied for a new transmitter location, then that station must begin operating at the new location by the time the construction permit expires.
If all goes well, and the station complies with the construction permit, then the station is granted a license, which usually lasts for seven years.
For more information on construction permits, check out the FCC's web site.