The current version of WSL is a set of API calls grouped into a compatibility layer, which maps (most) Linux kernel calls to NT kernel calls, along with a mostly-complete set of userland utilities. under the hood, what you're looking at is a full Ubuntu virtual machine, running under Hyper-V. This is, basically, "reverse WINE"—with all the pros and cons those Linux types among you who are WINE users will be immediately familiar with.
The next version of WSL, on the other hand, will essentially be a full-on Hyper-V virtual machine. WSL2 is available now in Insider builds, and will be generally available in Windows 10 version 2004. (Windows 10 Focal Fossa, anyone?)
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The major differences between WSL2 and a standard Hyper-V VM revolve around integration. There are no guest drivers to install, and the C: drive (along with any other Window drive letters) is already mounted read-write for you, under /mnt/c. You can easily and seamlessly read and write files across operating system barriers, without needing to think too much about it.
The WSL2 system will also be available on-demand more rapidly than you can expect a standard VM to boot—Microsoft says to expect fewer than two seconds to elapse between clicking the WSL2 icon for the first time after a cold boot, and getting a fully responsive Bash prompt.