tagging sites are sites that let you "tag" or mark certain sites as something you've found interesting or useful. they're also called social bookmarks. they're mainly used so that people with similar interest can share and exchange interesting sites and articles. some webmasters use it as a promotional technique for traffic on their sites. to respond to the thread starter, i don't use digg and delicious, i don't know how, and I haven't gotten into it really, but would like some help on it. i've used stumbleupon.com and i've recieved 2600 uniques in around 5-6 days from it. pretty effective to tell you the truth.
Rojo, Technorati, Wordpress, I'm sure there are others. If your just interested in social bookmarking, I've noticed an increase in the number of users of Blinklist.