What is the www-data user?

 https://askubuntu.com/questions/873839/what-is-the-www-data-user

www-data is the user that web servers on Ubuntu (Apache, nginx, for example) use by default for normal operation. The web server process can access any file that www-data can access. It has no other importance.

From the base-passwd documentation (/usr/share/doc/base-passwd/users-and-groups.txt.gz):

Some web servers run as www-data. Web content should not be owned by this user, or a compromised web server would be able to rewrite a web site. Data written out by web servers will be owned by www-data.