There are several types of operating systems, each with its own advantages.
To begin, the "operating system" is the set of interfaces and controls through which the user interacts with the computer. Since we generally don't operate on a binary level, we need something that we can look at that makes sense to us so that we can actually use the computer. Operating systems were created to make this easier.
The most common operating system in the world is Windows. It has all kinds of versions. In some ways its advantage is in its ubiquity. If you are familiar with it, you can use the computers in almost any office in the world. It is also considered more user-modifiable than the second system, Apple.
Though it used to be known as Macintosh, and now has names of animals like Leopard, calling it Apple will communicate what you are trying to say most of the time. Some people consider it the most user friendly and the system that works the best "out of the box," though the user experience is more tightly controlled than in Windows.
A third and very interesting system is any open source operating system like Ubuntu. These are infinitely user-modifiable and generally are grouped under the name "Linux." These are entirely available to the public and are built on the premise that corporate entities controlling how people use their computers is inefficient and a bad idea so all the source code and applications are free and available to the public.