A proof-of-work is a piece of data which was difficult (costly, time-consuming) to produce so as to satisfy certain requirements. It must be trivial to check whether data satisfies said requirements. Producing a proof-of-work can be a random process with low probability, so that a lot of trial and error is required on average before a valid proof-of-work is generated. Peercoin and Bitcoin both use the Hashcash proof-of-work. While Bitcoin requires proof-of-work for security purposes, Peercoin instead uses proof-of-stake, with proof-of-work only facilitating coin distribution.