Two main types of ISO images

There are at least two main uses for ISO's. One is a bootable, live CD (or DVD) and the other is a ready to install, fully pre-configured application or server.

Bootable ISO CD or DVD

Let's touch on the first which is a Live-CD. In this case, you simply write the ISO to a standard CDROM, then reboot your machine making the new CDROM your primary boot device. Once rebooted, your PC will be running the application when it boots up. This allows you to try out all sorts of applications and servers while not having to dedicate any hardware. Once done, you simply reboot your PC making the hard drive your primary drive and it's back.

Installable ISO CD or DVD

The second is an installable CDROM. Again, you write the ISO file to a CD then you install the ready made application or server onto a PC which you will be dedicating for the task. This could be a high end graphics application, editor or even a very highly optimized video storage system. On the other hand, you might have downloaded a ready to run, pre-configured email server or web server or streaming server.

Authors are working hard to make these ISOs available for you. Go ahead, give them a try, give the authors feedback and doing so will help us all to have many options for years to come.