Mail merge allows users to produce multiple personalized documents like letters, labels, and envelopes using data from a spreadsheet or database. It has two main components: a form document that contains the template for the document, and a data file that contains the recipient information. To perform a mail merge, a user selects a form document, chooses a data file with recipient records, writes the form document and inserts merge fields, previews the results, and prints the merged documents. The mail merge wizard guides users through each step of the process.