I am new to Ubuntu. In fact, I just started using Linux.
My question is: What is a distro?
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.
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:
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.
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.
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.