Cluster bombs are small explosive bomblets carried in a large canister that opens in med-air, scattering them over a wide area. The bomblets may be delivered by aircraft, rocket, or by artillery projectiles.
The CBU (Cluster Bomb Unit) 26, which was widely used in Vietnam and Laos, is an anti-personnel fragmentation bomb that consists of a large bombshell holding 670 tennis ball-sized bomblets, each of which contains 300 metal fragments.
CBU's have a high dud rate estimated at 10 to 30 percent. This leaves a lot of ordinance on the ground to cause havoc with soldiers and civilians alike for years to come.