Decision theory is the science of choosing actions on the basis of uncertain information.
Bayesian decision theory is the subarea of decision theory where apriori information is encoded into a probability distribution. While this creates philosophical confusion for some, and clarity for others, the implementation implications of using prior probability distributions are overwhelmingly positive.
libbayes is a library for programming applications in C that involve a Bayesian decision theoretic component. Note libbayes is meant to assist in the efficient numerical implementation of Bayesian models. In cases where significant analytical simplification is possible, using libbayes will probably result in worse performance relative to a problem-specific solution.