An ISO image is what’s called a disc backup - an exact copy of a CD or a DVD disc. ISO archives are typically created as DVD backups, but they can also be made “from scratch” using a specialized editor like WinISO. An ISO image can only store a single track of data, so it doesn’t work for music CDs and a few more arcane disc types.

It may help to think of the .iso file like a kind of archive because it usually contains a number of files and folders. However, unlike other archives (e.g. ZIP or RAR), the ISO archive saves everything in uncompressed form. Some commercial disc archive formats like UIF file and DAA files also include file compression.

To extract an ISO file you will need to install one of the many ISO file tools. There’s a multitude of applications that support the ISO archives, but most of them are either very expensive or simply lack any defining traits that would let you make an informed choice. So which one should you pick? Depending on your goal, there are two applications that I would recommend.

First, if you only need to open the archive, take a look at 7-zip. It’s a free open-source archiver that can open ISO CD files. It will let you open the disc archive, but that’s it - 7-zip can’t burn, mount or make new .iso images. It’s a neat and simple application and it works both on Windows and Mac computers.

In case you want a bit more features you should probably install a fully-fetured ISO image software like PowerISO. Yes, it’s shareware, but the application will let you do nearly anything with an ISO or UIF file - open it, edit, load it in a virtual drive, convert to a different format, and burn to a physical disc. PowerISO only supports Windows, though.

To summarize, ISO is a widely known, open CD archive format that is supported by most CD/DVD applications. Some general-purpose archivers can also extract .iso archives, but they generally can’t do anything else with them.