Bill Graham, who is active in the Hadoop community and a Pig contributor, gave a very clear and detailed intro to Hadoop and outlined how it is used at Twitter:
His slides can be found here.
Bill is a terrific instructor; thanks so much for the lecture!
Awesome introduction of Hadoop, can we please have same kind of intro lecture on Twitter Storm. we really appreciate this effort from twitter and UC Berkeley.. bring Storm on.. Woohoo!!