The Joint Direct Attack Munition (JDAM) is a guidance kit that converts unguided bombs, or "dumb bombs", into all-weather precision-guided munitions. JDAM-equipped bombs are guided by an integrated inertial guidance system coupled to a Global Positioning System (GPS) receiver, giving them a published range of up to 15 nautical miles (28 km). JDAM-equipped bombs range from 500 to 2,000 pounds (230 to 910 kg). The JDAM's guidance system was jointly developed by the United States Air Force and United States Navy, hence the "joint" in JDAM. When installed on a bomb, the JDAM kit is given a GBU (Guided Bomb Unit) identifier, superseding the Mark 80 or BLU (Bomb, Live Unit) nomenclature of the bomb to which it is attached.

The JDAM is not a stand-alone weapon; rather it is a "bolt-on" guidance package that converts unguided gravity bombs into precision-guided munitions (PGMs). The key components of the system consist of a tail section with aerodynamic control surfaces, a (body) strake kit, and a combined inertial guidance system and GPS guidance control unit.

The JDAM was meant to improve upon laser-guided bomb and imaging infrared technology, which can be hindered by bad ground and weather conditions. Laser seekers are now being fitted to some JDAMs.
Why is JDAM so cost effective?
The Joint Direct Attack Munition (JDAM) is a guidance kit that converts unguided bombs to precision-guided munitions. There are several reasons why JDAM is one of the most cost effective weapons ever invented.

By using GPS navigation, the JDAM guidance kit can be extremely cheap, as low as $25k/kit.  It is likely cheaper than any other precision-guided weapons.

GPS navigation provides good targeting precision, typically within meters. Combined with blast radius of 500-2000 lbs unguided bombs, JDAM can effectively destroy any targets on the ground.

Because JDAM has navigation capability, pilots no longer need to precisely aim the bombers  against the targets. It allows a bomber to quickly and precisely hit multiple targets along the route.

Some JDAM guidance kits include targeting sensors, such as laser targeting. They can be used against mobile targets, which further increases the use cases for JDAM.

In summary, by combining dumb bombs with a guidance kit, JDAM becomes a very cost effective weapon system, and is widely adopted by the US and other military forces.