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.

• [EMAIL] – email address of the subscriber
• [FIRST_NAME] – first name of the subscriber
• [LAST_NAME] – last name of the subscriber
• [FULL_NAME] – first and last names of the subscriber 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
• [SUBSCRIPTION_ID] – Unique ID that identifies the recipient
• [LIST_ID] – Unique ID that identifies the list used for this campaign
• [CAMPAIGN_ID] – Unique ID that identifies current campaign

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