Linux distribution

Linux distributions (often abbreviated as distros) are made of the Linux kernel and a collection of applications. The operating system will be made up of the Linux kernel and, usually, a set of libraries and utilities from the GNU project, with graphics that come from the X Window System. Small distributions may not contain big window systems full of features like KDE or GNOME, but use small window systems like busybox, uclibc or dietlibc. There are more than three hundred Linux distributions. Most of them are still in development, constantly being improved and changed.
History.
Before the first Linux distributions, a user needed to be an expert on UNIX, knowing what libraries and executables were needed to get the system to boot and run.
Linux distributions started to form after the Linux kernel began to be used by people besides the first Linux programmers.
Early distributions included: 
SLS was not well-maintained, so Patrick Volkerding created a distribution based on SLS, which he called "Slackware"; released July 16, 1993. This is the oldest being developed.
People who used computers wanted to use Linux distributions as replacements to Microsoft Windows operating systems on the PC, Mac OS on the Apple Macintosh and proprietary versions of Unix.
Package management.
Distributions are normally split into packages. Each package has a certain application or service. Examples of packages include a collection of fonts, or a web browser.
The package is usually given as compiled code, with installation and removal of packages done by a package management system. 
Linux distributions usually contain much more software than Microsoft Windows or Mac OS X.
Popular distributions.
Well-known Linux distributions include: 
Tools for choosing a Linux distribution.
There are tools available to help people make the decision easier. 
Screenshots of common distributions.
A few screenshots of common distributions just after installation: