You can think of an ISO file like a box that holds all the parts to something that needs built - like a child's toy you might buy that requires assembly. The box that the toy pieces come in does you no good as an actual toy but the contents inside of it, once taken out and put together, become what you're actually wanting to use.
An ISO file is the same way. The file itself is no good unless it can be opened, assembled, and used. The most common way to make use of an ISO file is to burn the ISO file to a CD or DVD. It's a different process than burning an MP3 file or a document to a disc because your CD/DVD burning software must "assemble" the contents of the ISO file onto the disc.
ISO image files are often used to distribute large programs over the Internet due to the fact that all of the program's files can be neatly contained as a single file - the ISO file.
ISO images end in the file extension ISO. For example, the free Ophcrack password recovery tool (which contains an entire operating system and several pieces of software) is downloaded as the ophcrack-vista-livecd-2.0.iso file.
