I explained del.icio.us as easy shared bookmarks a while back. There is a lot more development going on, and the del.icio.us team is obviously watching their users. I first saw the "for" tag used/explained on Brad Feld's blog, and then it got implemented.
Richard explains its use, but you really need to click through and read his whole, much longer post on what social software means to him:
Today del.icio.us introduced the for: tag, which you use to “send” links to people. It has a “private” RSS feed—probably would have been smarter for them to require your del.icio.us username and password to download it, since it’s a) technically possible and b) more secure—and anything tagged with that, only people access to that username’s account can view all the stuff tagged as such. So, if you want to “send” me something, tag it asJust a Gwai Lo: Aggregator: For Tag
for:sillygwailoand I’ll find out about it.
Basic summary: tag stuff with for:[delicious-user-name], and that person will get private notification of that link -- the digital equivalent of tapping someone on the shoulder and telling them to "look over there".