Due to the way GNU / Linux and free software is developed there are thousands of applications, tools, games abilities out there. Most desktop systems have for example, an office suite, e-mail and internet related applications, games, tools etc.
Thankfully over the years people have put together what is known as a distribution, you can download or obtain a cdrom / dvd with all the essential components on one place, along with an installer and a way to download more programs from the internet.
There are now 1000’s of distributions out there some are specialised such as scientific applications others are general purpose.
Popular distributions include
Where as companies such as Red Hat have created a server OS and are one of the worlds leading GNU / Linux companies offering complete solutions plus technical support and training / exams.
However many of these distributions contain non free or proprietary software, for example firmware components to make for example bluetooth or built in wifi work properly.
To this end the idea there are distributions such as Trisquel that are designed to only have pure free software. There is nothing to stop you getting hold of non free components. Distributions such as Debian are long standing and are designed again to be purely free software. Debian also supports a wide number of CPU architectures.
Some of these distributions come in both desktop and server versions, noteable is Ubuntu and derivatives such as lubuntu, xubuntu. Ubuntu has recently expanded so that its operating system is also available on phones and tablets.
More recently with the concerns about internet privacy not just due to what was revealed by Edward Snowdon, but also the fact that many websites are tracking users, for various reasons, storing and perhaps even selling data based on what users look at, distributions such as Tails have been developed to mask users activity.