The ISO image of a GNU/Linux Distribution is a huge file, if it gets corrupted during the download or while copying from some place, your whole installation could go bad
The Linux kernel is widely used now a days to for a usable system, generally what people refer as an OS, which we prefer to call as GNU/Linux distribution. Linux itself is available as a Free Software as well as most of the GNU/Linux Distributions are also available under similar FOSS licenses.
Linux is an operating system, core kernel or the actual operating system. It was developed by Linus Torvalds. The first prototypes of Linux were publicly released later that year (1991) under the GNU General Public License, designed and drafted by Richard Stallman commonly knows as RMS the free software guru.