Ubuntu is an operating system that is developed by a worldwide community of programmers as well as by employees of Ubuntu's commercial sponsor, Canonical.

Ubuntu is based on the concept of free or open-source software, meaning that you do not pay any licensing fees for Ubuntu, and you can download, use, and share the operating system free of charge.

Being a Linux-based operating system, Ubuntu has a well-deserved reputation for stability and security.

Linux has always proven itself to be a workhorse server operating system, and this is where, up until now, it has been most widely used.

In 2007, 78 percent of the world's top 500 supercomputers were running Linux.

However, in recent years, Linux has also become viable on desktop and laptop computers, making it an option for individuals and businesses.

Ubuntu is generally acknowledged to be the most widely used version of Linux available, and Mark Shuttleworth, the founder and CEO of

Canonical, estimates Ubuntu has between six and eight million users.

How does Ubuntu compare to Microsoft Windows and Apple OS X?

While Ubuntu is free of cost, the term "free software" more accurately refers to the freedom to run the program for any purpose, to study how the program works and modify it to your needs, to redistribute copies, and to improve the program and release your improvements to the public.

Ubuntu also includes many of the programs used for everyday computing at no cost.

• Office Suite: OpenOffice.org, a full office suite with a word processor, spreadsheet, and   presentation software that can read and write in .doc, .xls, and .ppt formats and can also output to PDF, and supports the ISO standard for electronic office documents, Open Document Format. (Free training for OpenOffice.org is available at LearnFree.org.)

• Desktop Email Client: Evolution, an email program with a similar interface to Microsoft Outlook.

• Web Browser: Firefox, the increasingly popular Web browser.

• Databases: The two best-known open-source databases on Linux are PostgreSQL and MySQL, but commercialdatabases such as Oracle and IBM's DB2 are also available. There are also tools like Glom that provide an easy-to-use graphical interface for designing and editing databases.

• Others: Ubuntu's online Applications Guide lists some Ubuntu-compatible applications that allow you to edit images, listen to and manage music, edit and watch videos, read PDFs, connect to instant messaging services from MSN, AOL, Google, Yahoo, and more.

• Updates and bug fixes: Security updates and bug fixes for applications and the operating system are managed by Ubuntu, and users are notified about these updates through an icon in the taskbar, which they can click on to install.

Another way in which Ubuntu differs from Windows and OS X is in the way it releases new versions.

Whereas Apple releases a new version about every 18 months to two years, and Microsoft took nearly five years between Windows XP and Windows Vista.

Ubuntu makes a new version available every six months, which users can update over the Internet without reinstalling the operating system, programs, or settings. (By contrast, neither Windows nor Apple offers online updates, and both require the purchase of a CD/DVD to install.)

Each release includes bug fixes and security updates at no cost for 18 months. After 18 months, security updates and bug fixes will no longer be provided, but you're free to keep using that version of Ubuntu if you like, or update online (free of charge) to a newer version that is supported in this way. Moreover, every two years, Ubuntu releases a version that provides bug fixes and security updates for a longer period of time — three years on desktops or laptops, and five years on servers — making it a good solution for those who want a longer rest between releases.

Of course, being open source gives Ubuntu one other major difference over Windows and OS X, and that is the ability for users to modify it in any way that suits them. There are two types of modifications most relevant here:

• Bug fixes, security fixes, or feature enhancements, which are contributed back to Ubuntu or the original application authors if relevant.

• Customizations to Ubuntu for a given set of circumstances, called a derivatives.

Some examples are highlighted below, but a full list can be found on Ubuntu's Web site:

• Ubuntu, a security-testing platform.

• Ubuntu Studio, for multi-media editing and creation.