It’s not a trojan or even a virus but a coin miner. They use the processing power of your device to mine bitcoins and sometimes other crypto currencies to generate revenue for the website. The often raise your CPU usage to much higher levels than usual. This won’t harm your PC in the short term but over time, they can reduce the lifespan of your device’s processors. For coin hive, just closing the tab will stop the mining. However, others can be more persistent. I recommend adding a coin blocker extension like *nocoin* to your browsers.