What is a Katyusha rocket?

I visited the Katyusha page on Wikipedia, and learned that Katyusha is a term for a wide variety of rockets loosely based on a Soviet design from the 1930’s.

I would like to know more about the specific design used by Hezbollah in southern Lebanon. Are they truck mounted as well? Are the rockets imported from countries like Iran and North Korea? Or is it a more primitive, improvised rocket, manufactured in workshops with available materials? I would think if it was truck mounted in large arrays as described in Wikipedia, it would be easier to spot from the air and destroy.

Who’s got the SD on Katyusha rockets?