I'm not sure if you want file sync or backup software but for file sync, there's our good friend Robocopy.
robocopy sourcedir destinationdir /mot:5 /r:50 /w:10 /zb /j /sj /copyall
The reason we like Robocopy here is that we can resume incomplete copies and control many aspects of how the copy will run. /mot:5 says "If you see changes in this directory space, run this command again in 5 minutes." This can be useful if you don't mind it running all the time, but you can omit it and just run the sync on a schedule as well. You might also want to use the /mir switch if you want a mirror of the current folder state instead of just copying over new data without deleting anything (i.e. the sync destination can keep things that aren't on the source any longer).
xcopy sourcedir destinationdir /h/i/c/k/e/r/y/d
is also a good tool for one time one way folder copying since Robocopy has more command switches than any three other shell commands in Windows.
Syncthing and Resilio Sync are other good options in this space and both of them have NAS clients that work in this space and work very well.
In the past, I suggested AllWay Sync as a Windows application that could keep folders synchronized for less technical users.
Beyond that, Acronis is $40 a year now if you don't already have licenses. Macrium Reflect is free from OldVersion.com. You have options in that space if you want to use a traditional backup application on Windows, although you might need to do a little scripting to set up file rotations for your incremental or differential backups.