We give an introduction to the Galois connection Inv - Pol between
sets of relations and sets of functions on a finite set A.
The Galois closed sets of functions are the clones and the closed
sets of relations are the relational clones. For a set $\Gamma$
of relations, the closure InvPol $\Gamma$ is the set of all relations
that can be defined from relations in $\Gamma$, using only
existential quantification and conjunction. This yields the
connection to the CSP: If Pol $\Gamma_1$ is contained in
Pol $\Gamma_2$, then CSP($\Gamma_2$) is polynomial-time reducible
to CSP($\Gamma_1$).