The first iteration of the Devastator (as seen in Dune II and 2000) that was used during the first war between Harkonnen, Atreides and Ordos was a massive tank with a fixed turret carrying two plasma cannons. It was extremely slow and vulnerable to Sandworms but had thick armour that allowed it to withstand plenty of returning fire from enemy weapons. It was also capable of self-destructing to cause a massive explosion in an attempt to take out nearby enemies.
The second iteration of the Devastator (as seen in Emperor: Battle for Dune) was used during the War of Assassins by House Harkonnen. Here, the Devastator is depicted as a "mech" that walked on three legs. Like its predecessor, it too was a slow, heavily-armoured vehicle capable of self-destructing and was armed with plasma cannons, but it also carried a missile launcher for use against lighter and more mobile threats such as infantry and Ornithopters. It was also capable of being transported by advanced, militarized versions of the Carryall, which made up for its extremely slow movement.