Picasa Web Albums makes it easy to share your photos with the world -- or you can choose to keep your photos as private as you'd like. Use album visibility options to control the privacy of your albums. Set your album visibility during the upload process and change them at any time in Picasa and Picasa Web Albums.
Anyone on the web can view public albums. Public albums can be found on your public gallery, in web search results, on your Google profile, and in Google+.
Limited, anyone with the link
These albums are visible to anyone with the link. They're technically visible to anyone, but they include a unique authorization key in the web address; the key is a combination of letters and numbers that make the address difficult to guess.
Limited albums are visible to the people you specify. Google+ users: The people you share with have permission to reshare your album. Within Google+, they'll also be able to see who else you've shared with.
The highest level of privacy, these albums are visible only to you. Changing an album to 'Only you' will wipe everyone off the album's 'Shared with' list.
Google Apps users: Picasa Web Albums also has album visibilities that allow you to share photos and videos securely within your domain.