If you want to understand the technology in a nutshell, think It's like a Wiki but it uses tags like del.icio.us.
Here are the help pages for the Nexi Library engine, also used at www.allabout.com. If you're like me, you'd rather learn by example than just read a reference guide. So, find a page that does something you like, then click edit this page and see how it does what you want.
Remember -- everything, even the links between pages -- is part of the page itself and can be edited.
N.B. my ego demands that I say that the technology Nexi is based on predates both wikis and tags, having been developed in 1994, but that's not really important. You just need to understand how it works.