What is the difference between test.website.com vs website.com/test

I'm trying to understand what is the difference of lets say test.website.com over the usual website.com/test .

For example Eurogamer uses this technique for its forum (https://community.eurogamer.net/ ) but everything else is in pages (https://www.eurogamer.net/news)

How is this technique called and what is its purpose ?

Thank You in advance