Win32 Disk Imager for Windows is a portable open source program to write disk image files with the file extension .img to removable media. Removable devices compatible with the software are for instance USB sticks and drives or SD cards.
The application has been initially designed to copy Ubuntu images to USB drives, but has quickly evolved from that to go beyond the initial design goal.
The program displays a simple interface on start up. First thing you need to do is select an image file in a supported format, img is the only one, that you want to write to disk. It is important that the file format is correct, as you will run into all kinds of issues otherwise.
In addition, make sure that the size of the disk image does not exceed the available disk space of the device you want to write it to. Since Win32 Disk Imager does not display those information to you, it is necessary to perform manual checks to make sure of that.
Next thing to do is pick the device that you want the image written to. All connected devices are listed in a small menu on the right of the image file that you have selected.
Select the driver letter of the removable drive after selecting the .img file on the local hard drive. A click on the Write button writes the img file to the removable drive. If the img is bootable, it will create a bootable USB drive. That's handy for Linux distributions and tools that come with booting options.
Img is not a format that is as widely known as iso for instance. Most Linux distributions, including Ubuntu, are now offered as ISO files instead. And for those purposes, an ISO to USB software like UNetbootin serves the purpose just fine.
Still, I happen to come upon an .img file occasionally and a tool like Win32 Disk Imager comes in handy in those situations.