A self-propelled explosive artillery rocket
Originally Soviet made and used during the Second World War, Katyushas are typically fired from mobile launch sites, which makes it very hard to track and destroy the launchers.
Katyushas can carry incendiary, fragmentation or in theory chemical and biological warheads. They have a range of between 14000m and 20000m, but they are not particularly accurate.
Katyusha has become the generic name for this sort of short-range truck-launched missile, and countries such as Iran now produce them and suppy them to Hizbullah
Untill May 2000, Northern Israel came under regular Katyusha attack from Hizbullah in Lebanon. After the Israeli withdrawal from Lebanon they stopped, but have resumed again in the Eastern Galilee and Har Dov region.
much of the facts from: http://www.clw.org/atop/atn/atn0500.pdf.