Allows the site admin to "subscribe" any number of email addresses to receive emails when articles in selected categories are published or modified. And as of version 5.3, you can also use the Joomla! Task Scheduler to send your emails as well, provided you have the Community Builder Auto Actions plugin or you know how to create a Task yourself.
You can create up to 1000 subscription groups. Each subscription group associates one or more article categories with one or more email addresses. Once the subscription group is enabled, any time an article is published or a published article is saved to one of those categories (Group Enabled = 'Save'), an email is sent to the list of subscribed addresses. The email subject and body can contain placeholders for things such as the article title, category, article intro and full text, link to the article on the site, and modification date. Some of these tags only work in the email subject.
The Notifier Task plugin allows the administrator to use the Joomla Task Scheduler to periodically check to see if a single article in one of a subscription group's categories is ready to be emailed. This plugin uses the subscription groups specified in the Content Notifier Joomla Plugin, so using this plugin requires that one to be installed. Only subscription groups for which Group Enabled is set to 'Task' are checked by this plugin.
The idea behind this plugin is not to give your site users the ability to subscribe to individual article categories but, rather, to use an external email list system to give the administrator the ability to configure which email list(s) receive what content. User subscription, then, lies within the email list system and this plugin just provides the "glue" between Joomla and that email system. Of course, you can also just subscribe specific individual email addresses to a group. It's all up to you where to apply the glue.
Back in the Joomla 1.5 days, newsletter extensions were few and far between. I used one called VEMOD News Mailer, which had a batch scheduler that alleviated the 250/hour sendmail limit my hosting provider imposed. It worked great, and the idea was to keep all the news on the site, just send it to subscribers when it was posted, similar to subscribing to a blog. The problem with any PHP-based newsletter extension is that, unless it comes with a handler that a cronjob can invoke, sites with low traffic volume don't generate enough events to make the scheduler do its job in a reasonable amount of time. They also put the processing burden on the site front end, slowing down the site users' interaction with the site.
This is what made me seek-out an external email list system that I could integrate with Joomla. I found Dada Mail which, although it has its own web interface, behind that is a Perl-powered system with batch scheduling and subscriptions that can be stored in a MySQL database. The database became the medium of exchange between the Joomla site and Dada Mail, and I just had to write a plugin to provide the "glue," a common thread with these two plugins. The Dada Mail Subscriptions Plugin was originally written for Community Builder (CB) because there were no custom profile fields in Joomla yet, nor did Joomla have the extensive and refined event system CB had. Since then, I have switched my sites over to using the Mailster Joomla! component, which completely integrates the email list system with the site and can be run using an external cronjob.
Both the Dada Mail and Mailster Subscriptions Plugins allow users to subscribe to individual mailing lists by using the list addresses to subscribe to Content Notifier subscription groups. All this "glue" is transparent to the site users, but together the two plugins create a pretty powerful newsletter system.