In PhpBB people can subscribe to Sub-Forums (eg. Questions, Your website, Forum, ...) by clicking this link:
Sub-Forum.tiff [ 6.33 KiB | Viewed 777 times ]
(Note: Sub-Forums, not the entire Forum)
They can subscribe to topics
Topic.tiff [ 10.49 KiB | Viewed 777 times ]
Or they can get notified when a reply is posted to topics they participated to
Topic-Reply.tiff [ 4.75 KiB | Viewed 777 times ]
Note that in all cases the user has to actively choose to subscribe to them. It doesn't happen automatically.
To me it's better that it doesn't happen automatically because otherwise I would get too many e-mails each day and see them as spam not as being useful.
I know there are plans to implement a feature where people can subscribe to the entire forum or watch the entire forum with an RSS feed. However that's not yet implemented in the current version of PhpBB. I would really enjoy the RSS Feed...