Let's start with a little (but fun) history lesson. Linux was first released waaaayyyy back in 1991 by Linus Torvalds. Speculation is that when he created it, he based the name on a combination of his name and that of another operating system called Unix. So what you wind up with is Linus + Unix = Linux. For the first 10 years approximately, Linux had no graphical interface, meaning no icons, no mouse to click on things, everything had to be typed manually and precisely at a command prompt at the terminal. Now, I realize that referring to operating systems and terminals might seem a little confusing, so let me try to bring these very simple things to light so you can think of them as naturally as breathing.
An operating system is, in simple terms, a piece of software that controls the hardware through a series of commands. In the past (and still today on some much larger computers), everything had to be typed in: "the command prompt." Since about 1985, operating systems (referred to as OS) have had a graphical interface. What that means is that everything is visually represented using icons, drop down menus, and mouse to point and click on programs we want to use. With this graphical interface (from this point forward, referred to as a GUI, Graphical User Interface) things just seem to happen by magic because all the commands are done automatically based on what a person clicks on. The commands are still there, they're just hidden from users because it's not necessary for the average person to know them. When Linux was created, it only had the most basic ways of doing things which meant no GUI. This did mean that it was often too difficult for the average person to use (without an advanced knowledge of operating systems and computers). Now that Linux does have a GUI, a person can use it the same as Windows with the option of still using a terminal.
Terminal software emulates the old hardware terminals (from 1970's and earlier, much earlier). This was only way to “talk” to a computer which looked like an old tube TV but couldn't do anything besides display characters such as numbers and letters. No pictures, no video, no nothing (and that includes fun).
Commands that are hidden from the GUI are available in the terminal. The caveat is that spelling counts, along with punctuation, and lower or upper case on all letters, known as “case sensitive.” Also, there is no leeway for mistakes. If you type in a valid command, but it's not the one you want, it could delete an entire hard drive (been there, done that), destroying memory (also been there, done that), and the worst yet, destroying a motherboard, which is for the most part, the computer itself (alas, poor Yorick, I knew him well, sigh). It is for these reasons that only the most knowledgeable and experienced use the terminal.
Linux is 100% free and generally found to be as easy to use as Windows and more stable. And, Windows keeps crashing, whereas Linux rarely does. It is very user-friendly: you use a mouse to point and click on any program to run it just like in Windows. Nearly all the software available in Linux works exactly the same as in Windows. The most common version of Linux for the RPi is Raspbian. There is also Ubuntumate (which I use) and a media center version of Linux called LibreElec. Other versions have been made workable, but that is for a far more advanced user.
Next we'll be looking at what makes everything work, that scary thing called LINUX.