  Let $M$ be a closed, oriented manifold of dimension $d$. Let $LM$ be the
space of smooth loops in $M$. Chas and Sullivan recently defined a product on
the homology $H_*(LM)$ of degree $-d$. They then investigated other structure
that this product induces, including a Batalin -Vilkovisky structure, and a Lie
algebra structure on the $S^1$ equivariant homology $H_*^{S^1}(LM)$. These
algebraic structures, as well as others, came under the general heading of the
"string topology" of $M$. In this paper we will describe a realization of the
Chas - Sullivan loop product in terms of a ring spectrum structure on the Thom
spectrum of a certain virtual bundle over the loop space. We also show that an
operad action on the homology of the loop space discovered by Voronov, has a
homotopy theoretic realization on the level of Thom spectra. This is Voronov's
"cactus operad", which is equivalent to the operad of framed disks in $R^2$.
This operad action realizes the Chas - Sullivan BV structure on $H_*(LM)$. We
then describe a cosimplicial model of this ring spectrum, and by applying the
singular cochain functor to this cosimplicial spectrum we show that this ring
structure can be interpreted as the cup product in the Hochschild cohomology.
