There are Hundreds of distributions of Linux. This will give you an idea of some of the bigger ones.
1. Low cost: You don’t need to spend time and money to obtain licenses since Linux and much of its software come with the GNU General Public License. You can start to work immediately without worrying that your software may stop working anytime because the free trial version expires. Additionally, there are large repositories from which you can freely download high quality software for almost any task you can think of.
2. Stability: Linux doesn’t need to be rebooted periodically to maintain performance levels. It doesn’t freeze up or slow down over time due to memory leaks and such. Continuous up-times of hundreds of days (up to a year or more) are not uncommon.
3. Performance: Linux provides persistent high performance on workstations and on networks. It can handle unusually large numbers of users simultaneously, and can make old computers sufficiently responsive to be useful again.
4. Network friendliness: Linux was developed by a group of programmers over the Internet and has therefore strong support for network functionality; client and server systems can be easily set up on any computer running Linux. It can perform tasks such as network backups faster and more reliably than alternative systems.
5. Flexibility: Linux can be used for high performance server applications, desktop applications, and embedded systems. You can save disk space by only installing the components needed for a particular use. You can restrict the use of specific computers by installing for example only selected office applications instead of the whole suite.
6. Compatibility: It runs all common Unix software packages and can process all common file formats.
7. Choice: The large number of Linux distributions gives you a choice. Each distribution is developed and supported by a different organization. You can pick the one you like best; the core functionalities are the same; most software runs on most distributions.
8. Fast and easy installation: Most Linux distributions come with user-friendly installation and setup programs. Popular Linux distributions come with tools that make installation of additional software very user friendly as well.
9. Full use of hard disk: Linux continues work well even when the hard disk is almost full.
10. Multitasking: Linux is designed to do many things at the same time; e.g., a large printing job in the background won’t slow down your other work.
11. Security: Linux is one of the most secure operating systems. “Walls” and flexible file access permission systems prevent access by unwanted visitors or viruses. Linux users have to option to select and safely download software, free of charge, from online repositories containing thousands of high quality packages. No purchase transactions requiring credit card numbers or other sensitive personal information are necessary.
12. Open Source: If you develop software that requires knowledge or modification of the operating system code, Linux’s source code is at your fingertips. Most Linux applications are Open Source as well.
The main "disadvantages" of Linux are primarily social / political, not technical. Because Linux is not as widespread on the desktop market, manufacturers are reluctant to produce drivers for relatively common pieces of hardware, and won't release technical specs so others can write drivers without restrictive NDAs, which violate the spirit of freedom that Linux wants to support.
For similar reasons, software developers often don't make their programs available on Linux. If a person or company comes to rely on a certain program, then they can't switch to Linux, despite Linux's many advantages.
Finally, there is a certain [perceived] lack of user-friendliness of desktop Linux distributions. Most GUIs on Linux work very differently than they do on more common interfaces like Windows or OS X. If a person is not used to the interface, they will not know how to accomplish a task and will thus consider Linux "hard" to use.
I switched to Linux several years ago, it does take some getting used to, but it was the best decision I made.