JAGS is Just Another Gibbs Sampler. It is a program for analysis of Bayesian hierarchical models using Markov Chain Monte Carlo (MCMC) simulation not wholly unlike BUGS. JAGS was written with three aims in mind:
1) To have a cross-platform engine for the BUGS language
2) To be extensible, allowing users to write their own functions, distributions and samplers.
3) To be a plaftorm for experimentation with ideas in Bayesian modelling