The simplest type of fission, it consists of two pieces of 235U such that separately their masses are less than the critical mass (this is the minimum mass corresponding to the minimum size sphere to maintain fission) but combined greater than the critical mass. These are detonated by bringing together very quickly, usually a high explosive is detonated firing one piece at the other.
The most well-known example of a uranium atomic bomb was detonated over Hiroshima, Japan on August 6th 1945, killing over 200,000 people. The energy released by a nuclear bomb takes several forms: blast and shock in air, thermal radiation (including light), immediate nuclear radiation (neutrons and gamma rays) and delayed nuclear radiation (radioactive fallout). This causes radiation sickness.