A mail merge is a capability of database software, word processors, and some e-mail programs that take a standard form and formats that form with unique fields such as e-mail address, name, address, phone number, or other personal information to make the message look unique. For example, a standard form may look like the example below.
We would like to thank you [username] for purchasing our product.
With a program capable of mail merge, the program would pull the personal information and place the appropriate information into the above fields. So, the above form may look like the example below.
1234 some street
City, State Zip
Dear Bob Smith,
We would like to thank you Bob Smith for purchasing our product.