The Linux kernel, together with GNU software and other freeware utilities, make up the operating system we usually call Linux.
Linus Torvalds, a bright young student studying at the University of Helsinki in Finland, released the source of the Linux kernel around 1991. He invited others to collaborate and contribute to its development.
In time, the Linux kernel matured and reached version 1.0 around 1993. The complete operating system together with an installer came out as distributions. Slackware was one of the early distributions of Linux available in 1993.
Today, there are hundreds of distributions available, each with its own legion of users.