JDAM stands for J
unition. These are designated GBU-29, GBU-30, GBU-31 and GBU-32 (GBU is U.S. Military speak for General-purpose Bomb Unit).
From the Federation of American Scientists at http://www.fas.org:
The Joint Direct Attack Munition (JDAM) GBU-xx is a tailkit under development to meet both USAF and Navy needs, with the Air Force as the lead service. The program will produce a weapon with high accuracy, all-weather, autonomous, conventional bombing capability. JDAM will upgrade the existing inventory of general purpose and penetrator unitary bombs, and a product improvement may add a terminal guidance seeker to improve accuracy. JDAM can be launched from approximately 15 miles from the target and each is independently targeted.
The FAS has good stuff on military systems; I recommend the site highly. The four variants of the JDAM listed above are for use with 250, 500, 1000 and 2000 lb. gravity bombs, respectively. They are tailkits, which means that they bolt onto an existing dumb bomb, and using their movable fins and navigation/guidance systems maneuver the bomb onto its target. They were conceived and built with the intention of counteracting the negative effects of weather, winds and smoke on conventional bombing accuracy.
The JDAM is told at the start of the mission what coordinates it is intended to hit. This makes it a Competent Munition. It is then launched (dropped) from an aircraft, either Naval or Air Force, up to 15 miles from the target. The JDAM uses internal INS or GPS systems to determine its present location, and maneuvers the bomb onto which it is bolted to strike the desired target coordinates.
One advantage of the JDAM is that it is quite cheap when compared to more complex guided, seeking smart weapons. It is also a large force multiplier for aircraft; whereas previously a bomber with 10 conventional munitions typically would expend all 10 in an effort to ensure the destruction of one target, a bomber with 10 JDAMs can allocate them to 5 or ten separate targets with a reasonable assurance of destroying perhaps five to seven of them, and in ideal cases all ten.