Some websites inject your PC with a cryptocurrency mining script to tap into your system’s CPU cycles. It is used as an alternative to advertising (or sometimes in conjunction with it) to make money off of your visits, though not all sites are upfront about the practice. Worse yet, a hacked website could be using your CPU unbeknownst to everyone,

Source: Hot Hardware – Politifact Fact Checking Website Hacked To Mine Monero Cryptocurrency