What is the point?
Sharing information about web sites of interest. Storing bookmarks on the "cloud' instead of on the hard drive of their computers.
How does it work
How secure is it
Users set their own passwords. Their site is as secure as their passwords
What else can you do?
Diigo allows users to create self defined groups among their contacts
Where can I find out more?

http://www.diigo.com/

Diigo tutorial
http://23things.barrow.wikispaces.net?Diigo+Tutorials?f+print

Social bookmarking
http://www.commoncraft.com/bookmarking-plain-english