  Let G be a group which is topologically a CW-complex, BG a classifying space
for G, and A a discrete abelian group. To a central extension of G by A, one
can associate a cohomology class in $H^2(BG,A)$. We show this association is
injective, and bijective in many cases. A homomorphism to G lifts to the
extension iff the pullback of the associated cohomology class vanishes.

Algebraic Topology