On a Linux system there are users and groups. Users can belong to certain groups and usually default to belonging to a group with the same name as there user name.
This allows you to make files and chmod them in such a way (664) so that only the owner (me), someone belonging to a certain group (by default - me) can read and write to that file, while everyone else can only read that file.
Where cPanel comes into it I have no idea. It is simply a web application used to aid in server administration.
Web servers however run as a specific user and group. Apache for instance might run as the apache, and the apache user might by default belong to the apache group. Now, because php executes as an apache module (part of apache), if you create a file using php it will belong to the apache user and the apache group.