What do people mean when they say they "rolled" a certain class?

its an old term deriving from Dungeons and Dragons, where you determine your starting attributes using Dice. CJ 11:09, 6 Jan 2006 (EST)

Rolling for green items in 1.80 has changed. all rolls are now done after everyone has selected Need or Greed.

Is it known how blizzard generates the rolls? Is it truly random or only algorithm-based?