You define in each computer you install Dropbox a "Dropbox folder" (it doesn't need to be the same in every computer) and every file and folder inside the folder will be synced.
Example - Computer A - Linux
Example - Computer B - Windows
C:\My Dropbox\folderA\In this examples, the folder folderA and every file and folder inside, and the file fileA.txt, are synced between both computers.
Only the files and folders inside your "Dropbox folder" are synced, but if you create a symbolic link to a file or folder outside the "Dropbox folder" it will also be synced.
Create a symbolic link
$ ln -s '/storage/outsidefolder/fileB.txt' '/home/youruser/Dropbox/insidefolder'The fileB.txt and the folderB will be synced into your "Dropbox folder" and synced into the the "Dropbox folder" in Computer B.
$ ln -s '/storage/outsidefolder/folderB' '/home/youruser/Dropbox/insidefolder'
Check how to create symbolic links in Windows here.
If you try this between two similar Linux computers, the files in the second computer will not be created in the same outside folder. It will stay in the "Dropbox folder" of that second computer.
fileB.txt in Computer A: /storage/outsidefolder/fileB.txt
fileB.txt in Computer B: /home/youruser/Dropbox/insidefolder/fileB.txt
If you know a way to do this please let me know by commenting in this post or by sending me an email :-)