Merge tags are tags that are replaced before sending out the message. The format of the merge tag is the following: [TAG_NAME] or [TAG_NAME/fallback] where fallback is an optional text value used when TAG_NAME is empty.

• [FIRST_NAME] – first name of the subcriber
• [LAST_NAME] – last name of the subcriber
• [FULL_NAME] – first and last names of the subcriber joined
• [LINK_UNSUBSCRIBE] – URL that points to the preferences page of the subscriber
• [LINK_PREFERENCES] – URL that points to the unsubscribe page
• [LINK_BROWSER] – URL to preview the message in a browser

In addition to that any custom field can have its own merge tag.