ANFO stands for Ammonium Nitrate / Fuel Oil, and describes a crude but effective explosive that is used by farmers to clear stumps and by the mining industry (because it is easy to pump as slurry) to break up overburden rock and expose ore in open pit mining.
Its formulation is well known. Ammonium nitrate is a substance most commonly used as a fertilizer. When mixed with diesel fuel it can be used as an improvised low yield explosive. Commercial explosives based on ANFO are also available.
Because of the easy availability of its ingredients, ANFO is often used by terrorists for large bombs. ANFO was often used by the Provisional IRA in its bombing campaigns and Palestinian extremist groups. It was also found in the Manila apartment which Ramzi Yousef occupied after an apartment fire led authorites to his apartment, which was a bomb-making factory and plotting center for Operation Bojinka.
So called "fertilizer bombs" were made famous in America by their use in the Oklahoma City bombing which maimed and killed hundreds of victims. The bomb used was only remotely based on traditional ANFO bombs though It used a sophisticated triggering sequence and compressed oxygen to achieve a larger detonation than is possible using mundane means.