  In this expository paper we give an elementary, hands-on computation of the
homology of the little disks operad, showing that the homology of a $d-fold
loop space is a Poisson algebra. One aim is to familiarize a greater audience
with Euclidean configuration spaces, using tools accessible to second-year
graduate students. We also give a brief introduction to the theory of operads.
New results include identifying the pairing between homology and cohomology of
these spaces as a pairing of graphs and trees, and treating the cooperad
structure on cohomology.
