One's worship system consists of the focus of his attention, be it God, status, exaggerated scarcity (like money), himself, or something entirely imaginary like a view of one's past.
A worship system can be evaluated by how much time, energy, and worry one places in something on a daily basis.
Belief systems may simply be an imprecise descriptions of underlying worship systems.
Atheism is described by many as a belief (or disbelief) system, but it typically entails self-devotion, censorship of criticism of evolution, and opposition to prayer. As such, atheism is more a devotion or worship of certain (anti-Christian) values than an objective set of beliefs.