Linux is a open source and free software. That means basically that everyone can see and edit the source code, it just must be as free available as it was before.
The free software movement was launched in 1983 with the primary goal of developing free software replacements for the proprietary software. (Free Software Portal). In 1991 Linus Torvalds, a student of informatics, wrote a central component of a new operating system: the Linux Kernel. He offered it for free on the internet and encouraged people to test and improve it.
Free Software is mostly licensed under the GNU General Public License (GNU GPL)

Linux is a Unix-like computer operating system. This means that it is similar to Unix, but written in a different source code.
A core security feature in these systems is the permission system. All files in a typical Unix-style filesystem have permissions set enabling different access to a file. The more security you add, the less ‘friendly’ an OS is to use.
As Unix, Linux uses small, specialized programs working cooperatively, so any Linux distribution contains a large number of individual programs. Almost every program written for Linux has a man page, which documents the command syntax and every command option, and to be universally available. Whatever you want to know, there is a way to find it.

the upsides

the down sides

Want to know more about Linux?
Beginners: Learn Linux A beginners guide to Linux for those with little or no computer experience.
An Introduction to Linux
What is GNU/Linux?
Linux is Not Windows
Why Linux is reliable
The Cathedral and the Bazaar
GNU licenses

The Origins of Linux - Linus Torvalds (85min)

Get help – report bugs – contribute

Search the documentation first: Xubuntu, Ubuntu, Debian, Linux
Xubuntu is a derivate of Ubuntu, which has a larger community and therefore more help resources. Ubuntu is based on Debian.

Official Ubuntu Documentation
Ubuntu Tutorials & Tips
recent issues can be found in the The Ubuntu Forum Community
Unofficial Ubuntu guide
or use a search engine of your choice

Read about Ubuntu Forums and ask your question there

Communicate with other users

We use Xubuntu.

Contribute to Xubuntu

1 Introduction to Linux, A Hands on Guide

2 Getting Started with Linux