How to Sync(permanently) two folders without using symlink?

Let me explain my doubt clearly. I have two folders, say /home/testuser and a root folder / I hv these two folders in sync all d tym. But these two are not related by symlink. There are no symlinks between these two. But yet they are related. If i create a file in / i can see it in /home/testuser. If i delete a file in /home/testuser it gets deleted in / too. It looks like they both are same folders with diff names. I need to create a similar structure in different server, so i need understand these. Please help me.