One sees the word ‘cult’ thrown around quite often and the word has deeply pejorative connotations. People who belong to mainstream religions are comfortable calling some minority religious groups cults but the distinction between a religion and a cult is elusive. Is Scientology and cult or a religion? How about Mormonism? How about the Moonies? How about the Branch Davidians or the Sai Babaists?
I too have tried to find a distinction but failed. The best that I could come up with is that a religion tends to have the properties of being old, having many members, and some political clout while those groups that are called cults tend to lack any or all of those qualities. But since there were no real differences in the nature of the beliefs involved, I concluded that there is no substantive difference between a cult and a religion.
But via Pharyngula I saw this graphic that I think captures the essential difference.