Syncthing is a file synchronization program. It allows you to sync files between computers over LAN or the Internet. It is a very simple program that just gets the job done.
I use it to synchronize files between two computers and my phone. When synchronizing two computers, I find it to be a much faster—and smoother—approach than doing it with a USB, while mantaining my privacy and avoiding the exposure of private document to companies on the Internet. I also use it to back up a couple of folders from my phone, including my pictures. Every once in a while I will activate syncthing on my phone, it will sync everything with my computer and then I just exit it. It is fast and simple.
If you want to synchronize/back up your computer or phone privately, check it out!