I was initially intrigued by the Raspberry Pi for its size, power and WiFi capability. It looks like they have come out with a 4th version that has dual mini-hdmi ports. I like the Pi, but do not plan on ever needing to run two screens using it. The reason I was initially into the Pi was for its ability to run Debian Linux and the cool packages that distro offers; Apache webserver, SQL database and other functions. I have been a long-time developer of LAMP systems; Linux, Apache, MySQL and PHP, but having a credit-card sized computer doing it was an amazing thought.
Linux
There are too many different flavors of this Operating System to count now. I initially was learning Redhat 6.0 in the early 2000's, but now exclusively run the Debian version of the Raspberry Pi OS. Not a lot is different and even being gone from the system style for over a decade, not much changed.
Apache
This webserver application, run mostly on Linux operating systems, has been the long-time running champion for percentage of website utilization throughout the net. It's an easy to configure program, but you really have to get through the strife of learning Linux first.
MySQL
This is now called "Maria DB" for the fact that MySQL sold out. Basically you need a SQL server in the background to assist with the website database references. It makes it a lot easier if its a local SQL server than on another computer.
PHP
PHP is a hypertext preprocessor that is a programming language for the server to interpret. I initially started using PHP on version 4, just as version 5 was about to come out. After about 6 years, it is now on version 7 and honestly completely different, in a good way. It makes working with a database a lot easier.
If you want to learn programming or website production, get a Raspberry Pi and get a LAMP server set up. You can use the Keezer Kiosk to get it done, just don't install the Keezer files. There are also other programming languages you can learn like C++, Python and CGI, so check it out.
You can also connect the Raspberry Pi to sensors and other functions for more use. You can check out the Raspberry Pi community pages for other projects that were done with this little computer.