From Wikipedia, also includes a brief bit on "Redemption", the prequel TV movie to Season 7:
Season 3
Set 3 years after season 2, season 3 begins and ends at 1:00 PM. While struggling with a heroin addiction, Jack must re-infiltrate a Mexican drug cartel in order to acquire a deadly virus being marketed underground. Jack must then stop the mastermind behind the virus from releasing it.
Season 4
Set 18 months after season 3, season 4 begins and ends at 7:00 AM. Jack must save the lives of Secretary Heller (his new boss) and Heller's daughter Audrey Raines (with whom Jack is romantically involved) when they are kidnapped by terrorists. The same terrorists then launch further attacks against America, and Jack is forced to use unorthodox methods to stop them, methods which will have long-term consequences for both Jack and the U.S.
Season 5
Set 18 months after season 4, season 5 begins and ends at 7:00 AM. Jack is believed to be dead by everyone except a few of his closest friends. Terrorists with connections to the U.S. government attempt to steal nerve gas in order to protect U.S. oil interests in Asia, and Jack must resurface to stop them as well as dismantle the government conspiracy.
Season 6
Set 20 months after season 5, season 6 begins and ends at 6:00 AM. Jack is released after being detained in a Chinese prison for twenty months. Terrorists plot to set off suitcase nuclear devices in the United States and Jack must stop them; later, Jack has to prevent sensitive circuitry from falling into the hands of the Chinese to prevent war between the U.S. and Russia.