<p>I have heard the 'shard' technique mentioned several times with regard to solving scaling problems for large websites. What is this 'shard' technique and why is it so good?</p>

Karl Seguin has a
good blog
post about sharding.
From the post:
Sharding is the separation of your
  data across multiple servers. How you
  separate your data is up to you, but
  generally it’s done on some
  fundamental identifier.