The basics:
Simply naming a repository ".github.io" will cause github to automatically turn it into a website. The url will be https://.github.io and it will simply display your README.md file in nicely rendered markdown. If you have an index.html file in the repo, it will display that instead.
- So to make a website, simply create a repo with that specific name and do whatever you want for an index.html file and it works.
"Project sites":
You can also turn any other repo into a website. For these other repos with any old name you choose, you have to go into settings and just check that you want it in "github pages". Then it will do the same thing as before where it looks for an index.html or displays your README. This time the url will be https://.github.io/<repo_name>
- So you can mix and match and turn as many repos as you want into websites all centered around your main url.