Simplenews builds on the following basic concepts.
Subscribers subscribe to newsletters (categories). That connection is called a Subscription. Nodes of enabled content types are newsletter issues. These are then sent to the subscribers of the newsletter the issue is attached to.
Sending is done by first adding a row for each subscriber to the mail spool. Then they are processed either immediatly or during cron runs. The actual sending happens through a source instance, which is first instanciated based on the mail spool and then used to generated the actual mail content.