The Grad began life as a Soviet multiple rocket launcher system called BM-21 in 1963. It is sometimes referred to as a Katyusha, as Katyushas were the first Soviet multiple rocket launchers manufactured during WWII. The Grad was first fired at Israel from Gaza in 2006.
The Kassam is an indigenous Palestinian product named after Hamas's Izzadin al-Kassam Brigades. The first Kassams from Gaza hit Israel in 2002.
While not a missile with pinpoint accuracy, the Grad is much more accurate than the Kassam.
While a Grad has a range of 18-30 km; a Kassam is limited to 16 km. A Grad carries a 20-30 kg. explosives payload, while a Kassam carries just 10 kg.
A Grad is 122 mm, in diameter, compared to the Kassam's 170 mm.
Cities within range of a Grad in the South include Netivot, Kiryat Gat, and Ofakim. A Kassam can strike Gaza envelope communities and Ashkelon.