Theists are of the opinion that the probability that god exists is likely, in say 50-100%
Agnostics are of the opinion that the probability that god exists is uncertain, in say 33-66%
Atheists are of the opinion that the probability that god exists is unlikely, in say 0-50%
One gotcha is that, if you account for the greater number of theists, the distribution is not symmetric. Atheists tend to be less certain (more agnostic) than theists.