A link (or URL) is an address to a resource. For public links, this means an address to a file or folder on your account.
Public links can be shared with anyone via. e-mail, twitter, facebook or instant messaging/chat. Since the link is only a few characters long, it is a great way to share large files.
You can make a public link, that is ready to be shared with friends or colleagues, by selecting a file or folder, and pressing "Share". Enable public access and click "Copy URL".
You can also create links for photos and albums in Jottacloud Photos in the same way.