Public information is visible to everybody, including random visitors, web crawlers, search engines, etc. By nature, all stamp type and issuer information is public. Also, if a member's item has been publicly displayed by that member in an exhibit at a stamp show, or published in a magazine or book, then it should be normally be made public as well.

Members-only information is visible to all logged-in StampData members, but not to the general public. This class of visibility is useful for internal discussion, for instance when evaluating whether an item is genuine or a forgery. If all your friends are on StampData, they can all see your material, but it doesn't get put on the net at large -- or indexed by search engines.

Private information is visible only to yourself. Its very existence is filtered out of everybody else's view of StampData contents, with a couple minor exceptions - summarized collection data for a type mentions the existence (but not the number or owners) of private objects, and admins may review private information. (StampData is not a Swiss bank! If you have stolen stamps, don't record them here!)

If you're the owner/uploader of objects, images, or notes, you always have the option of changing their visibility status as you like. Keep in mind that public information will eventually be collected by search engines, and after that there may not be much point in making it more private.

Also note that in the case of rarities or specialized collecting areas, others may be able to guess that, say, a public image of a private object means that the private object is in your possession.