.MIG files belong to one of two Microsoft tools that facilitate the transfer of data from one computer to another. Both User State Migration Tool (USMT) and Windows Easy Transfer facilitate the transfer of data between two Windows computers. Windows Easy Transfer lets you transfer User Accounts, Emails, Documents including music and picture, Internet favorites and more. All items selected during the backup process will be saved to a .MIG file which can then be imported on your new computer using the Windows Easy Transfer tool.
Whilst USMT is designed for large deployments, Windows Easy Transfer is aimed at home / SOHO users. Due to its properties, USMT 4.0 can also be used to streamline user migration and large-scale deployments of Windows 7.