If you're just starting out, you may want to know where Linux came from. In the 1970's there was a kernel called UNIX. Then in 1984, a guy named Richard Stallman founded the GNU Project. His goal was to create a completely UNIX-compatible Operating system completely out of free software. In 1987, Andrew S. Tanenbaum released a UNIX-like system called MINIX. Altough the source code was available, Modification and distribution were highly restricted. Then in 1991, Linus Torvalds started work on a free replacement for MINIX. That would eventually become the Linux Kernel. Linux is completely free and anyone can modify it and make there own flavor. There are hundreds of flavors out there, but covering them all would not be practical.
Pictured is Linus Torvalds, maker of Linux.