In MMORPGs, monsters have a list of items they can drop when they are killed. A player can get 0, 1 or more items from the same monster, but a dice is rolled for EACH of the dropable items.
WoW: To get Zod's Repeating Longbow, one knows Lady Deathwhisper is the only one monster to be killed. The game rolls a 1000-sided dice, and if 136 or below comes out, the bow is dropped. However, one can also drop the Nibelung from Lady Deathwhisper at 15.6%. Hence, approximately 2.1% of the time, the dragon will drop both the Longbow and the Nibelung, and maybe other things as well.
RO: To get an Ice Pick, one has to kill Eremes, LoD or RSX, but they drop it at respectively 15%, 0.1% and 0.05%. But Eremes also drops Glittering Jacket at 90% or Exorciser at 35%. So 4.7% of the time, Eremes will drop at least these 3 items.
Roguelikes generate a lot of their content procedurally. Unlike MMORPGs, roguelike drops are most of the time randomly-generated: monsters do not drop specific items at certain percentage, they drop classes of items.
Diablo (and Diablo II): the drop system consists of Treasure Classes (sets of items) and a drop process. When a player kills a monster, a first random number is used to determine which TC will be dropped. For Andariel, some of these TC are armo18 (1.02%), weap69 (0.017%) or nothing (28.35%). All monsters have the sum of their TC = 100%. In other words, 28.35% of the time Andariel will drop nothing, and the rest of the time, it will drop one item. Once the TC has been chosen, another random number comes out for "the odds". "The odds" is the chance a particular item is chosen from a TC. For the weap69 TC, the Eldritch Orb has 1/15 chance to be selected. In the end, a player has around 0.001% chance to drop exactly the Eldritch Orb from Andariel. But Baal also drops the weap69 TC at 0.54%, so Baal can drop Eldritch Orb at 0.54% * 1/15 = 0.036%. Finally, various elements such as the item property (fire, ice, ...), name prefixes/suffixes or durability are generated.
Team Fortress 2: the Item Drop System used to give (at 25% chance) a weapon to the player every 25 minutes of game. To avoid unlucky streaks that players complained about, Valve changed it: every time an item drops, the time of the next drop is determined.