Linux is a KERNEL, NOT AN OPERATING SYSTEM started by Linus Torvalds in 1991 at University of Helsinki in Finland. Linux was at first just a hobby for Linus. Linux is free software.
It just so happened that just when Linux was getting usable, GNU had everything it needed except a kernel. Thus, people started using Linux with GNU to make a completely free UNIX-like operating system called GNU/Linux.
Today Linux and GNU/Linux have come a long way from the early 1990's, although they're still mostly used by geeks.
A copyleft free software license. That means that anyone can do anything with something under the GPL except deny others' rights given by them by the GPL. Also the GPL requires that all copyright notices and notices thatrefer to the GPL stay.
What all computers were before GUIs came about. CLI is an interface that only uses text. Some stupid people and groups (like micro$oft, apple and linspire) try to get rid of CLI taking a lot of power away from users. CLI is still used a lot on UNIX and UNIX-like OSs.