There are many factors:
A lot of users take the "ease of use" into account and may favor Ubunu or Mint.
Others may prefer package managers such as RPM, APT, or pacman and may choose Fedora, Debian, or Archlinux, respectively.
Still others may like having full control over every aspect of their system and thus may like Archlinux, Gentoo, or Slackware.
Sometimes it'll be about application: Phone? Android. Server? Red Hat. Desktop? Ubuntu. Router? dd-wrt.
It all boils down to preferences and needs.
Haswell
Yes. BackTrack is a Linux distribution that focuses on penetration testing and computer forensics.
Yes.
No, Ubuntu is part of Linux.
Ubuntu is a Linux distribution (as in a specific OS setup with the Linux kernel.)
Yes.
Scientific Linux is a Linux distribution. It is a free and open source operating system and aims to be as close to the commercial enterprise distribution as possible.
A server with a Linux distribution installed on it.
Many Linux distributions are intended for home users.
No Linux distribution is currently made for any PalmPilot model.
Linux Security Tool Distribution (STD) is really good.
SUSE Enterprise Linux. They also contribute heavily to OpenSUSE.
Linux Distribution