ISO is a file extension used in burning DVDs. Like

Filename.iso

More info here.

ISO Definition

Linux operating systems are distributed as free downloads of files with this kind of extension and your DVD burner has to be able to burn this kind of file.

There are programs that do this out there for free.

Alternatively your local Tesco or Asda sells magazines such as 'Linux User'. Issue 141 has a DVD containing a bootable DVD of Linux Mint 17 which, if you like it, can be used to install the system on your machine. This cost me £5.99 in my local Asda. A good bit cheaper than Windows.