Race banks were collections of usually biological materials that preserved the genetic structure of a species. They took different forms depending upon the race preserved and the technology of the society that constructed them.
The Minyan approach to the concept was extremely compact. All the vital information was stored in two golden cylinders aboard the P7E. Because Minyos and all her inhabitants were destroyed, Captain Jackson of the R1C led a hundred-century quest to find the P7E and retrieve the race banks. With the help of the Fourth Doctor, Leela, and K9, he was ultimately successful. However, he also discovered that his crew were not, in fact, the last survivors of his race. Instead, the "Trogs" and other inhabitants of the planet which had coalesced around the P7E were in fact descendants of the original crew. Thus, although his quest was successful, Jackson ultimately discovered that the contents of the race bank were not strictly necessary for the perpetuation of his race. (TV: Underworld)
The Kastrians had a race bank located in the thermal chambers on their homeworld Kastria where "a hundred million" crystal particles were stored, waiting to be brought to life by the regenerator chamber.
When Eldrad returned to Kastria after a one hundred and fifty million year absence, he hoped to restore his species from the race bank and rule over them as king as they set out to conquer other worlds. However, he found the bank to be empty. A recording produced by the former king, Rokon, before his death, explained that, in anticipation of Eldrad's return and in fear that he would wage "eternal war throughout the galaxy", the Kastrians elected to destroy the race bank as well as themselves. (TV: The Hand of Fear)