A root is the starting point at which branches form.
A root folder is the main folder that contains all other subfolders/files. It is the highest point in the hierarchy. When a root folder contains subfolders, it can be called a folder tree.
The web root folder is the folder that is publicly accessible on the Internet. It typically has some sort of index/default page, but it certainly isn’t a requirement.
Each domain/subdomain has its own web root. For a particular domain where a webpage/website is hosted, the web root can be named any number of different things, depending on how the host is configured, such as:
- htdocs
- www
- wwwroot
- httpdocs
- http_public
For your MySite.com example, the httpdocs folder is that domain’s web root.
As far as visitors are concerned, they never see the web root’s folder name or where the web root folder is located on a server. They only have access to the contents of the web root folder after visiting a domain (or IP address, in some cases).