Alongside is an apt description. It feels a bit like running Linux in a Docker container.
The Windows Subsystem for Linux (WSL) has it's own directory in the Windows filesystem that corresponds to /. It's uses some NTFS magic to store the Linux file attributes that don't directly correspond to NTFS file attributes.
It also, within the Linux environment, mounts your host drives under /mnt.
For what I generally use the OSX terminal for, WSL probably hits about 80-90% of my use cases. A lot of lower level utilities have weird issues - e.g., 'ip addr' seems to present the Windows network interfaces as though they were typical Linux ones, 'ss -a' gives a bunch of netlink errors, dmesg says "dmesg: read kernel buffer failed: Function not implemented", 'tcpdump' doesn't work, etc. On the other hand, curl, scp, ssh, etc do exactly what you'd expect.