This is the editor for any fields that accept HTML (i.e. group and event descriptions) as well as when sending emails.
This button lets you add personalization variables (where available). This replaced the old "Personalize" dropdown.
This allows you to insert a previously designed email template.
Open the document library using this button (more details on the new library below) to add files.
This button lets you upload a file to be included as a link in the email (will not be saved to the document library).
This button makes the highlighted text a link.
This button lets you upload an image to be included in the email (will not be saved to the document library)
For those with HTML knowledge, clicking on this button will let you edit the underlying HTML giving you full control over the content structure/design.
As you type, you'll notice the content area expands to the height of the text. This means you'll always be able to see the entire content and no scrollbars are needed anymore.
Finally, if your cursor is on an empty line, a circular plus sign will appear to the left of the editor. This is a "quick insert" and lets you add content on that line. The same options here can also be found in the toolbar of the editor.