Well, I suppose it depends on what you mean by 'open'. While an ISO file can be opened by any number of archival or compression programs, it is not really designed for that. An ISO file is intended to be burned to a DVD or CD, most typically to create a boot disk. To burn an ISO to a disk, you will need a CD or DVD Burner (a drive that both reads and allows for burning), a blank writable disk and some burner software, like Roxio or BurnCDCC.
So if you download an ISO file of a Linux distribution like Ubuntu, you would burn that file to a CD or DVD. Then you could boot from that disk to run a Live version of Linux (meaning run in memory and not installed on your hard drive). Or you could run the installer on that disk to install it to your hard drive. (Warning: doing the Install will destroy your Windows install.) Or a movie might be downloaded as an ISO to burn to disk. Or a bootable password recovery program. So, ISO is just the file format of a CD or DVD. That's all. So look inside it if you want. But burn it if you want to use it as designed.