A d100 is a 'probability die' often used in RPGs like Dungeons and Dragons. Also often found noted as d% (percentile dice), it mostly clearly denotes a chance percentage coupled to a certain probability. A d% is rolled for several different occasions. A few examples:
-Looting a creature slain by tabletop knights often requires the d% to make out the actual loot
-Encounters with creatures are often governed by a d% roll. The higher the roll, the harder (usually) the encounter.
As previously noted, a d100 is mostly 'rolled' by the use of two d10's, each with different colors or sizes, where initially agreed upon what die governs tens and what governs units. Also previously noted, a physical d100 is rare and often large.
A physical d100 is an invention made by a man named Lou Zocchi; it is a spherical die upon which the numbers 1 through 100 are written. Though multiple attempts at d100s have been made earlier, the invention of this 'Zocchihedron' as it is called in 1985 has been lauded the birth of the first 'agreeably honest' d% die. Noted however is, that it still isn't as completely integer as two d10's. Truth be told, physical d% dice are a lot more expensive and mostly considered collector's items. I possess two, and I think of them like this, too. Other dice often used in RPGs are d3, d4, d6, d8, d10, d12 and d20, though also known are d5, d16, d30. There are even d7's and d24's.