I came across this rather nicely presented tut on the basics of using Libpcap for writing your own sniffing apps

hxxp://reactor-core.org/libpcap-tutorial.html

If any of you fancy learning more about using Libpcap in writing your own security apps I can highly recommend this book

"Building Open Source Network Security Tools: Components and Techniques"
Mike Schiffman
Wiley Publishing
ISBN 0-471-20544-3

I hope this info is of use to some body