- Shared folder. The recipient needs to install Dropbox (a barrier) and gets access to an entire directory on your computer. If she overwrites a file, your file gets changed too. This option is secure.
- Public folder links. You can send links to files in your public folder to other people. Anyone who has the link can access the file. If someone has your Dropbox user id, they could guess the name of a file, since the URLs are predictable. It is still hard, but not very secure. The recipient does not have to install Dropbox.
- Link sharing, the new one. It is now possible to get a sharing link for any file in Dropbox, not just the public folder. The link is garbled and very hard to guess. People can decide to view the file online (useful for mobile devices), or download it. The recipient cannot change the file, but if you do, the link points automatically to the latest version. If you want you can remove the link from Dropbox and other people can no longer access the file anymore. There is no need for the recipient to install Dropbox. Very useful.
The details are explained here.