Ubuntu is a GNU/Linux based open source operating system created and supported by Canonical Inc. Ubuntu is an African Bantu word which nearly means “Humanity towards others". It was launched in 20 October 2004. Ubuntu is based on Debian package management system and uses Unity Shell interface in their latest versions of operating system. Unity is based upon GTK3 framework. In past Unity was meant for Ubuntu Netbook Edition. Later it was taken as Ubuntu’s main GUI shell. But we can install Gnome3, KDE or dozens of other desktop environment or window manager as we like. Ubuntu is based on another GNU/Linux distribution known as Debian. 

The main and biggest USP (Unique Selling Point) of Ubuntu is its community and support. Ubuntu was the first Linux distribution to offer commercial support on desktop Linux version. Ubuntu is free to download, install, use, modify and pass on to friends or to customers. According to Mark Shuttleworth the creator of Ubuntu, Ubuntu Operating System is the most widely used Linux distribution in the world. Ubuntu comes in various flavors. Official flavors include Kubuntu, Xubuntu and Lubuntu. There are so many unofficial flavors too some of them are Backtrack - a security distribution, Linux Mint etc. 

Mac Operating system is developed by Apple Inc. Mac OS is based on Mach kernel and NextStep operating system. Apple was founded by Steve Jobs. Mac OS is closed source with some open source and BSD components. It is based on UNIX principles and technology. Mac OS is knows for popularizing graphical user interface. Mac OS from the beginning tried to minimize user work and increase performance. Mac offers graphical menu even at low level. Mac makes use of copyrighted ROM to achieve this. 

Mac OS initially only supported Motorola and PowerPC platforms. With version 10 codenamed Lion Mac OS supports Intel Microprocessor family too. MacOS ships with their own hardware only. When we buy PC we buy hardware from one company and software from another company. That is not same with mac. Mac is more popular among all types of artists. Mac OS is compatible with Windows. You can transfer and edit windows documents and settings. Many mainstream softwares and games such as Microsoft office, Adobe Suit, Age of Empires are also available for Apple Mac OS too. Apple provides a great and reliable ecosystem for developers so that they can develop a core foundation of their software and port it on all Apple products. 

Microsoft Windows operating system is an closed source proprietary operating system created my Microsoft Corporation, USA. You have to purchase license in order to use windows operating system. You can also buy preinstalled windows in laptops or computers. Microsoft Windows uses a controversial hybrid kernel type in all of their products. Windows was started with Windows version 1.0 and the current stable version is Windows 7. Windows was basically knows for its graphical user interface which gave it fame and major market share back in Windows 95 days. 

Windows is many times criticized of using monopolistic misbehavior such as implementation of Digital Rights Management or driver signing and not fixing bug and security holes in time. Windows was created by Bill Gates when he was student. Later he left the education in order to earn money by selling windows operating system. Windows today is knows as patent troll in the computer industry. 

Irrespective of all this negative points windows keeps dominating the desktop computer market today. Windows is praised and liked by common day to day user for their graphical interface and easy to user applications. Linux and Mac OS are strong competitor to Windows OS in coming future. 

Finally, these above top three operating systems are most popular and mostly used operating systems...