I've been using Linux (and Solaris, BSD, HP-UX, Irix, etc) for nearly 20 years.
In the Debian tutorial is clear: everything ending in d is a Daemon.
Well, many *programs* that end in d are daemons... for example, crond or dhcpd. Not all daemons end in d though, for example sendmail. And certainly many things end in d which are not daemons, for example sed or ld. Things that end in .d are NEVER daemons, but are configuration directories for the most part... For example, /etc/profile.d has nothing to do with daemons. In fact I would probably go as far as to say that most directories ending in .d are not daemon related.
In my /etc, these are not daemon related: bash_completion.d, dev.d, gre.d, ld.so.conf.d, libpaper.d, logrotate.d, lsb-release.d, makedev.d, modprobe.d, pam.d, profile.d, readahead.d, reader.conf.d, rwtab.d, sane.d, setuptool.d, and yum.repos.d
And these ARE daemon related: cron.d, init.d, rc*.d, netplug.d and xinet.d
I suspect .d is for "default."
Daemons, under the Linux File System Standard, will generally be located in /sbin or /usr/sbin, and while many daemon program names end in d, many do not. While daemon CONFIGS will be in /etc, no daemon or other program should be in /etc (not counting executable scripts).
daemon is a"Disks And Extensions MONitor"
Uhh, no. That is absolutely not true. A daemon is a background system process that does stuff, and comes from the Greek word for servant.
Just a quick glance at that Debian help file tells me you ought to go read the TLDP stuff instead. Not that the Debian file is wrong, but its not very clear and has some things that will be confusing to newbies.
You can tell much more about what kind of file a program is by where it is. The filesystem has a pretty clear structure that is defined in a standards doc. Search the net for Linux filesystem standard.