Section Headings (under the Edit Newsletter section, below the Preview section of the page.)
Newsletter Subject - subject line of the email that will be sent out.
Newsletter Header - the body of your email message. It begins with a "Dear FIRSTNAMEHERE" tag that will use the first name of each user to whom the e-mail is sent. Any html tags that are usable in Articles are usable here.
Included Articles - article ID numbers that you include here (separated only by commas--no spaces between numbers.) Headlines and photos for each article will appear in your newsletter immediately below the body text in the Newsletter Header.
Newsletter Footer (optional) - a sub-heading that shows up in bold below the headline and in a smaller font size than the headline; limited html tags are allowed here.

In Depth

Tips
1. Leverage work you have done in creating your articles: use that text and html layout work in your newsletters, too.
2. Newsletters with one highlighted event are more effective than ones with many events. Many organizations find that their event communications and advertisements are much more effective when they contain only one (1) event per newsletter/e-mail. You certainly have the ability to send an e-mail that contains as many event announcements as you would like, but you will probably get better results when you constrain it to only one.
3. Short summary lists of events work well. It is very effective to have a short summary list of upcoming event announcements by using the "Included Articles" feature to have that list below the text body summarizing the one event you are highlighting in a communication.