The umask controls the default file creation permissions, 0002 means files will have 664 and directories 775. Setting this (by editing the umask line at the bottom of /etc/profile in my case) means files created by one user will be writable by other users in the www-group without needing to chmod them.
I've looked that up, but I am told:
# The default umask is now handled by pam_umask. # See pam_umask(8) and /etc/login.defs.
And here I am blocked as I don't know pam_umask, I want to change the umask to 0002. How to?