As a registered user, you can "subscribe" to topics, or even entire forums, if you subscribe to a forum, every time that a topic is updated, or a new topic is created inside the subscribed forum, a notification will be sent to the email on file. The same goes for a topic-only subscription, you will be notified when a topic for which you have a subscription is updated.

Under the hood.

When a topic or forum is created or updated, the forum system internally "scans" and finds those users that are subscribed to either the topic or the topic container (forum), then it creates a record into a queue table. The forum system does not send out an email since notifying all users could potentially slow thing to a crawl. An internal script (python of course) exists in your application installation that is triggered via a cron job (either using web2py's embedded cron job or using the standard Linux crond daemon), the forum administrator decides how "often" this script should run, pyForum currently uses a 5-min wait between scans, a range between 5 and 10 minutes is generally acceptable, however for more sensitive instances, you could potentially trigger the script every time there is an addition or update.

-- sb