Yahoo! Mash is a new social network launched by Yahoo! sometime early September 2007 to compete with MySpace, Orkut and Facebook in offering mashups of existing web services on a single user interface. The service is currently in beta on an invite only basis.
Mash has the unusual feature of letting a user edit other peoples pages, unless the user turns this feature off.
Mash pages are made up of "Modules" which can be added by the user or their friends. One module that can be added to your page (or friends page) can show photos from Yahoo!'s photo sharing site Flickr.
As with other Social Networks, pages which are not yours can contain a common friends module to show any mutual friends you may have.
Source: Wikipedia