Here's some more info from MSNBC:
FIRST USE OF THERMOBARIC WEAPONS
NBC News has learned that for the first time, U.S. Air Force F-15’s used thermobaric weapons Saturday against caves south of Gardez. Two of the extremely accurate, 2,000-pound thermobaric bombs were dropped into caves in eastern Afghanistan where al-Qaida and Taliban fighters were believed to be hiding, U.S. defense officials said.
The thermobaric warhead is designed to penetrate deeply into caves or tunnels, creating many times the normal pressure to propel the explosives further inside the cave and kill more people. The weapons contain “fuel-rich” warheads that can fill tunnels with fireballs.
Pentagon officials have said they believe this weapon, called the BLU-118/B, will be more effective in destroying caves and tunnels than traditional explosives. Defense Department officials reported that a small number of the bombs were sent to Afghanistan in December, after a successful test in Nevada earlier that month.
Sources told NBC News that the current mission involves hundreds of Afghan fighters and about 200 U.S. troops from various armed services. The battle plan called for a combination of U.S. Special Forces and Army 101st Airborne assault troops fighting alongside Afghan allies, with U.S. bombing support from the air, said Pentagon officials, speaking on condition of anonymity.