The mail merge process entails the following overall steps:Set up the main document. The main document contains the text and graphics that are the same for each version of the merged document — for example, the return address in a form letter.Connect the document to a data source. A data source is a file that contains the information to be merged into a document. For example, the names and addresses of the recipients of a letter.Refine the list of recipients or items. Microsoft Word generates a copy of the main document for each recipient or item in your data file. If you want to generate copies for only certain items in your data file, you can choose which items (or records) to include.Add placeholders, called mail merge fields, to the document. When you perform the mail merge, the mail merge fields are filled with information from your data file.Preview, change individual documents, and complete the merge. You can preview each copy of the document before you print the whole set. And you can change individual copies of the document.You use commands on the Mailings tab to perform a mail merge.IMPORTANT   If you plan to use custom contact fields with the Contacts list in Microsoft Outlook or if you plan to use a Public Folders Contacts folder, you must begin the mail merge process in Outlook.
It is used for mass mailings and mass printing of letters or documents