Software
Raspberry Pi is a small computer that needs an operating system (OS) to operate just like you may probably have a copy of Windows OS installed in your computer. An OS is also software that supports and performs the basic functions of a computer.
There are multiple operating systems that you can install into your raspberry board, however, the official operating system is Raspbian. Raspbian is a Debian-based Linux distribution specifically designed and optimized for the low-performance hardware of the Raspberry Pi.
Third-party OS for the Raspberry Pi are available for download at raspberrypi.org/downloads.
If all of this is new to you, I encourage you to choose NOOBS.