Linked Folder in a shared folder in Dropbox
Most file systems know 2 types of linked files: Hard linked files and soft linked files (more here on SO and here on linuxgazette) By default, OS X and Windows use soft links (alias, symbolic links). Both types of links make it possible to acces the same folder from different locations.

Dropbox will treat each folder (original or linked) as an original folder on your system, as if it is the only folder with that content. Dropbox will not know if a folder is a symbolic link or not.

Modifications to the content of a linked folder will change the contents of that original folder. As I said, dropbox will treat that folder as any original 'normal' folder, so changing the content within dropbox will change the original.

In short. On each system there is only 1 original folder. You can create as many links as you like, the content of that folder only lives once on your system. Altering the content on any location (e.g. dropbox) will change the content of that original folder.