If you wanted to run Linux and Windows on the same machine, it could be a pain. You either had to set up a dual-boot machine or install a virtual machine. On modern Windows machines, Linux is an app like any other. Here's why that matters.
One reason Linux vaulted ahead of other competitors to Windows on the PC was that it was so easy to run it alongside an existing Windows installation by repartitioning the hard drive. This means splitting the hard drive into parts that will contain each operating system's file system. This is because different OSes, including Windows and Linux, handle files differently.
Switching between operating systems is easy at boot time. Most Linux distributions will install a bootloader, with GRUB being the most common. Most modern Linux installation programs can detect which operating systems are already installed on the computer when installing the bootloader. When you turn on your computer, it will display a menu of the installed OSes. If you want to run Linux, you choose Linux, and if you want to run Windows, you choose WIndows. Some people have more than two operating systems installed on their machines.
While it's easy to choose OSes at boot time, the disadvantage is that if you want to run a program that's installed on the other system, you have to reboot the machine. This might be more of a mental problem than a technical one. You have to interrupt what you were doing. If you had an idea for the app you were building in Linux while you were playing a game or surfing the web in Windows, you would have to wait for Windows to shut down and Linux to come up. You might lose your inspiration in the meantime.
The Windows installation can interfere with Linux, especially when updating or upgrading Windows.
One way to run both systems is to use a virtual machine, such as one set up in VirtualBox. You need a lot of memory and processing power to run a virtual machine at an acceptable speed. While you don't have to reboot, you still have to wait for the virtual machine to boot to be able to do anything with it.
Windows Subsystem for Linux lets you run Linux distributions inside an existing Windows installation without rebooting. It initially was a compatibility layer that translated system calls, or a special programming interface that allows programmers to access operating system functions, into Windows ones.
WSL was designed for programmers, particularly people who work with internet and web services. Most servers run Linux, and Microsoft had been losing market share to Apple laptops. macOS runs an environment similar to Linux systems, so developers of these apps can test them easily on their own machines. WSL is an attempt to bring that to PCs
WSL2 runs a real Linux kernel. The kernel is the real heart of the operating system. This approach is similar to running a full virtual machine in VirtualBox, but stripped down. You can also run graphical Linux apps, whereas the original WSL only ran textual command-line programs.
WSL has integrated Windows and Linux on the same desktop. If you lived through the '90s and the 2000s, this is a surprising thing to see. The advent of WSL would be the strongest evidence that the Gates/Ballmer era of Microsoft is over, where the latter called Linux a "cancer" and the company was dedicated to discrediting Linux and open-source software as much as possible in the tech and business worlds. That strategy clearly didn't work as Linux continued its long march through the world's data centers.
Now if inspiration strikes when you're doing something else on Windows, you can fire up a terminal and act on it. You don't have to wait for your system to reboot to try out a code idea. You can code in Linux while still being able to access games and mainstream applications in Windows.
One advantage of WSL is that you can access both Windows and Linux from each other. You can browse Linux directories in File Explorer and view Windows directories from the Linux terminal. If you have a graphical Linux file manager installed in WSL, you can also browse Windows directories that way. You would probably just use Explorer, but little "because you can" things are still fun to show off.
More practically, you can also run commands from each other's environments. You can run a Windows command from Linux and Linux commands from Windows.
For example, to run the Windows ping command from Linux:
All of this has the effect of blurring the distinction between Windows and Linux when you have WSL installed. Skeptics might wonder if this is a secret plan for Microsoft to take over Linux. The company is infamous for its "embrace, extend, extinguish" approach in its older days where it would take an open standard, add its own "enhancements," and make people dependent on them.
