Providing your project URL with anyone who isn't logged in as you will display the public version of that project. To find the URL to share, simply find the 'Share URL' button at the bottom of your project, detail or stats page.
There are some significant differences you'll notice with public projects. Firstly, they're read-only and cannot be added to, deleted, dragged or dropped. They are also cleaner in design, and allow for commenting, read more.
You can also share this URL on Twitter. In the sharing modal window, simply press the Twitter icon and follow the steps to post this to your feed.
Even if someone already has your URL, you can protect it at any time by adding a password. This locks your project and all Prevues within it from unwanted visits. To find out more about security, read this article.