A Linux distribution is simply a collection of a specific kernel with specific support programs. Most of the included software is free and open source software. There are many distributions designed for personal computers, but there are also Linux distributions designed for supercomputers or for a specific purpose, e.g. penetration testing (Kali Linux).
There are many distributions available (over six hundred), but two major distributions became the foundation for many other distributions: Red Hat and Debian.
Some of the more popular Linux distributions beside Red Hat and Debian are Ubuntu, openSUSE, Fedora, Mandriva Linux, and Arch Linux.