The point of origination of res judicata is a judgement in a former suit by the court whereas the estoppel is originated by the act of the parties themselves. The reason for the development of the doctrine of res judicata is for the public policy, which means an end to litigation, while the estoppel is rather based on the principles of equity, which means it bars the multiplicity of representations. On the other hand res judicata bars the multiplicity of suits.
Authoritatively, res judicata is a bar on the jurisdiction of a court, and is a basic test to institute a suit whereas, as mentioned before, estoppel is only a doctrine observed in evidence and disables the parties to speak further. In the lines of prohibitory values of both, both doctrines in one way or the other prohibits something, for which the res judicata prohibits a man averring the same thing in successive litigations whereas estoppel prevents a person from saying one thing which he has said before and consequences have arrived thereafter. And the last point of difference between both doctrines is the binding nature that can be showed by observing the doctrines in general. In res judicata, both the parties are subject to the application of res judicata since both has approached a subsequent court for the same matter to be adjudicated upon therefore its binding effect is on both the litigants. On the other hand, in case of estoppel, occurrence of a binding effect is only on the party who had made the previous statement or conduct through which the course of litigants have changed and thereby only that person will have to suffer the binding force.