d6 dice are a platonic solid, the cube. Six-sided dice are the most common dice type in gaming, used from casinos to board games to roleplaying games. There are, in fact, several RPGs that use nothing but the 6 sided dice for all rolls. Within the gaming industry the d6 comes as part of a standard 7-dice set with numbers on the faces, in sets of 12 with pips for numbering, and in sets of 36 smaller dice (12mm instead of the standard 16mm) with pips for numbering. The only way to get larger quantities of d6s with numbers instead of pips is to purchase them individually.
Casino grade 6 sided dice must be manufactured to strict and incredibly precise tolerances to ensure rolling as perfect as possible. The dice we use in RPGs are made through traditional plastic injection molding and tumbling, making them less truly random -- though close enough that it takes thousands of rolls before the difference becomes statistically significant. Close enough for gaming purposes, in other words.
The oldest cubical dice found are about 4,000 years old, found in Egyptian tombs. Six-sided dice have been found in ancient Chinese excavations and in thousand year old Viking burial mounds.