The Foundation site states "Beginners should start with NOOBS – New Out Of the Box Software."
To install either NOOBS or Raspberry Pi OS (previously called Raspbian) you need access to a computer with a SD Card writer; NOOBS installation requires more steps and is slower than installing Raspberry Pi OS and wastes SD Card space for the installer.
Downloads has links to the latest versions of both NOOBS and Raspberry Pi OS and installation instructions.
There is also a Raspberry Pi Imager for an easy way to install Raspberry Pi OS and other operating systems to an SD card ready to use with your Raspberry Pi.
You can purchase a pre-installed NOOBS SD card from many retailers which is suitable for those without a computer with a SD Card writer, but REQUIRES a screen (or TV) and keyboard (and preferably a mouse).
What is NOOBS?
New Out Of the Box Software (NOOBS) is not a conventional Operating System (OS) - it is an Operating System installer which is easy for beginners to use.
NOOBS enables users to install/re-install one or many OS and to configure which OS boots by default.
NOOBS contains Raspberry Pi OS and LibreELEC. It also provides a selection of alternative operating systems which can then be downloaded from the internet and installed.
Raspberry Pi OS vs NOOBS
If you only want to run Raspberry Pi OS (the officially supported Raspberry Pi OS) there is no benefit is using NOOBS.
Raspberry Pi OS can be set up to run in headless mode (without a screen or keyboard) - this is not possible with NOOBS.
Once setup NOOBS enables the user to install/re-install OS directly on the Raspberry Pi without any other computer access (but does require a network connection).
NOOBS does permit multi-boot, but if you want to do this I suggest you try PINN instead (this is an upgraded NOOBS which supports more OS and is a lot more flexible). On the other hand SD cards are cheap and 2 small cards are probably cheaper than 1 large card.
PINN comes without any included OS and requires a network connection for OS installation.
If you want to evaluate the different OS available for a Raspberry use NOOBS.
But as soon as you know which OS is best for you immediately install the plain OS without NOOBS. Why? If you have any issues with a NOOBS image and ask the community for help most of the time you will get no or very limited help. All the folks running a Raspberry in production and may be able to help you don't use NOOBS and will suggest to install a plain OS first.
Installing using Raspbian Iso is much easier as and saves your SD card memory too, rather than using NOOBS
Download the ISO file from the Offical Website Also download software such as Etcher, or Rufus to create a bootable drive
Format your drive, select the iso and extract the ISO file to your SD card