An IMG file format is, at least nowadays, most likely to be a CD or DVD image file, the same as say, an MDS or ISO file. In most cases, changing the extension of an IMG file to ISO will make it still usable in programs such as IMGBurn and Nero.
An IMG file can also be an archive format for disk images of floppy diskettes or full HDDs. The full contents of both can then be stored or transmitted.
Additionally, IMG is also the format of choice for Apple Disk Copy of the Mac OSX.
More technically, an IMG file contains a raw dump of all the content on a disk and is supported by many programs and all major operating systems.
Related glossary terms
Related software tools
ImgBurn carries on from where DVD Decrypter left off! (Well, for the burning part anyway!)
Very powerful CD, DVD and CD/DVD image file manipulator/data recovery tool
Nero Burning ROM (Shareware)
Nero is the best and most complete DVDR / CDR / Blu-ray all-in-one software