IMG files are “disk images” which are common on the Macintosh, but not on Windows. An IMG file contains an exact copy of every bit of a floppy disk, CD, or DVD. On the Macintosh, they can be “mounted” just as if you inserted a disk. On Windows, you need to use a program to open the IMG file. One such program is WinImage, which will extract the files from the IMG file in the same way you would extract the files from a ZIP, RAR, or StuffIt archive. Of course, if you have a Mac handy, you can copy the IMG file to the Mac, open it, copy out the files, and then copy them back to your Windows computer.
If your IMG file contains a DVD and you just want to burn it, you can save yourself some time by using ImgBurn, which can burn IMG files directly.