Some blurb is required here about what professional networking is and why it is important. As a starter: as a search term in Google it comes back with 134,000,000 hits!
On the Monster Jobs web
site there's seven links to short networking articles: content.monster.co.uk/section2077.asp
MySpace for grown-ups. FriendsReunited for professional colleagues, but free. Wikipedia description of LinkedIn: en.wikipedia.org/wiki/LinkedIn
Home page of LinkedIn: www.linkedin.com
The frequently referred to 10 Ways to Use LinkedIn: http://blog.guykawasaki.com/2007/01/ten_ways_to_use.html
Lots more links can be found at Lee Hill's "Living LinkedIn": http://backgroundknowledge.typepad.com/linkedin/Networking