The Raspberry Pi is a tiny single-board computer that includes a CPU, RAM, USB ports, an HDMI port, an ethernet port, and WiFi. The "disk" drive is a micro-SD card, and if you plug in a power supply, a USB keyboard, and a screen via HDMI, you can install a full-GUI Linux Debian environment. You can install PHP, Python, Java, etc., and use it for a huge range of projects. This thing is mind-bogglingly small - about the size of a pack of cards.
There is a vast amount of info about the Pi online, with tutorials, discussions, examples of things built with a Pi, etc. I'm not going to attempt to cover that - just google "raspberry pi" and you will see what I mean.
The cheapest place to buy Pi boards and accessories is from the home of the Pi: raspberrypi.org. You can buy a Pi board from about $35 US, a case from $5, etc.
The simplest way to get going in the Pi world is to buy a starter kit, like this one on Amazon.ca:
You can get a starter kit from many vendors, and it should come with at least a case, power supply, heatsink, and a pre-loaded SD card, as well as the Pi board. Assemble the kit, plug it in, and you have a working tiny computer.
There are a large number of types of Pi boards available, but for most people, the decision comes down to whether you want a Pi 3 or Pi 4. Once you decide, you buy the latest version of that model (currently the Pi 3 B+ and the Pi 4 B).
The 3 and 4 boards have different placement of the ports, so the cases are not compatible - you decide on the board and then buy a matching case.
Comparing the two:
For 25 years I always bought the biggest, fastest computers available, but recently I've become a fan of using the least power possible, so I got a Pi 3 B+.
When you are getting started with a Pi, the software you need on your SD-card is NOOBS (New Out Of the Box Software) which will come preloaded if you buy a kit, or can be downloaded from here. When NOOBS starts, it lets you install Rasbpian, which is a flavor of Debian Linux. You can choose to install a lite version with no desktop, a version with a GUI desktop, or a version with a GUI and lots of desktop applications.
The Pi needs to be set up as a desktop machine to start with, but once configured, you can easily tuck it away and access it remotely. Mine is connected to my router in a closet, but I can bring up a full GUI environment if I want to, using a laptop.
So, unfortunately, you do have to plug in an HDMI monitor and USB keyboard in order to get started. If you don't have a monitor, you could always use a TV set, or ask a friend. Once you get the Pi running, you can set the Pi to a fixed IP address (you can often do this just on your router) and install a remote-desktop server such as xrdp. Then you can unplug the monitor and keyboard, and whenever you turn the Pi on, you will be able to connect to it. Xrdp allows you to connect to the Pi's GUI from any laptop, using the Microsoft remote desktop client. You can get an RDP client on Windows, Mac or Chromebook. If your desktop is running Linux, then you don't need me to tell you how to connect to a Linux server!
Regardless of which version of Rasbpian you have installed, you can always chose to connect to the Linux command line on the Pi via SSH from any laptop.
If you are looking to learn Linux, I can think of no better way than having your own tiny Linux server. You can play around with commands, install and uninstall software, etc., free in the knowledge that you cannot do any harm. I certainly recommend making a copy of your SD card, so you can quickly restore a working OS!
You can set up a web server like Apache or Nginx, and set up a CMS or use the Pi as a test server for Node.js or Python code. If your network provider is ok with it, you can even put your web server on the internet, by setting your router to pass-through ports 80 and 443. This is not to be taken lightly, though, so for the most part it's probably best to have fun with your Pi as a local Linux server.