I am new to Ubuntu. In fact, I just started using Linux.
My question is: What is a distro?
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A Distribution of Linux is a package of software, designed to be a complete Operating System.
A linux operating system consists of a few parts:
System level software
The GNU* Userland
User interface software (bash, ...)
Compiler, linker, parser-generator, ...
Application software (emacs, bc, ...)
Which itself provides
Hardware abstraction software (such as Pulseaudio, ...)
Application Software (A word processor, a spreadsheet, a web browser, ...)
This Combination of Software, forming the complete operating system, is called the 'Stack':
A Distribution of Linux is made by combining all of those parts to build a complete and usable operating system for a specific purpose, or for general use.
An example of a general use operating system is Ubuntu. It comes in varieties for Desktops, netbooks and servers, making it useful to the general public.
An Example of a specialised Distribution is CHAOS, a Small OS designed to run on high performance computing cluster nodes.
In short, Imagine all of this stuff being bundled together, so that it can be easily distributed to people, rather than them having to build it all themselves.
Distributors often make design decisions on behalf of the user, according to what they expect the user wants.
Ubuntu is made to be fun and easy to use
TinyCore is made for people who want to use it on old hardware
RHEL is designed to run well on big supercomputers and critical servers
These decisions include the addition/removal of software (such as KDE vs. Gnome), and, potentially, the (re-)design and implementation of software (such as Ubuntu One).
There are a whole bunch of distributions of Linux alone, as illustrated by this (immense) diagram:
*: Technically, GNU is an Operating System; However, every OS needs a Kernel, and GNU's kernel (the Hurd) is not finished, so GNU uses the Linux kernel. Most people today call this combination a "Linux Operating System", though Linux is a small part of it. Arguably the most important part.
A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions (often called distros for short) consist of a large collection of software applications such as word processors, spreadsheets, media players, and database applications.