Great question — answering as the UphillTech team:
The .d stands for directory. It is a convention to distinguish the directory-based configuration from a configuration based on a single configuration file. Often you will have both in some capacity, for example /etc/logrotate.conf and /etc/logrotate.d/ . It is also usually the case that all (reasonably-named) files in such a directory are automatically combined into a single configuration. Packages can then install files into a directory like that, and they will be used automatically. Again, /etc/logrotate.d/ is a good example. By constrast, a directory of configuration files that does not end in .d probably just contains random collation of configuration files belonging to the same...
Full answer by Peter Eisentraut: https://superuser.stackexchange.com/a/230261 (CC BY-SA 4.0).
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