I do a lot of certificates for the 5th graders at the end of the school year, and it gets really difficult and time consuming when you have to fill out hundreds of these things by hand. Mail merge only asks that you have their names in columns on an Excel document, and that's it. It's great.
I really hate filling out certificate after certificate, so mail merge has been a life saver. What mail merge does is it links up a field from your document (either Word or Publisher in this case) to data from an Excel document. It's fairly easy, especially once you've done it a few times.
What I have here are examples of how to do this in a few different formats.
How To Mail Merge in Microsoft Word / Publisher 97 PDF DOCUMENT
How to Mail Merge in Microsoft Word 2010 PDF DOCUMENT
How to Mail Merge Video: