What is difference between res judicaga and res sub-judice

Res Sub-judice means "stay of suit"

As per section 10 of civil procedure code:

No Court shall proceed with the trial of any suit in which the matter in issue is also directly and substantially in issue in a previously instituted suit between the same parties, or between parties under whom they or any of them claim litigating under the same title where such suit is pending in the same or any other Court in India having jurisdiction to grant the relief claimed, or in any Court beyond the limits of India established or continued by the Central Government and having like jurisdiction, or before the Supreme Court.

Section 11 of CPC refers to res-judicata which is reproduced below:

No Court shall try any suit or issue in which the matter directly and substantially in issue has been directly and substantially in issue in a former suit between the same parties, or between parties under whom they or any of them claim, litigating under the same title, in a Court competent to try such subsequent suit or the suit in which such issue has been subsequently raised, and has been heard and finally decided by such Court.

Explanation I- The expression “former suit” shall denote a suit which has been decided prior to the suit in question whether or not it was instituted prior thereto.

Explanation II.- For the purposes of this section, the competence of a Court shall be determined irrespective of any provisions as to a right of appeal from the decision of such Court.
Explanation III.- The matter above referred to must in the former suit have been alleged by one party and either denied or admitted, expressly or impliedly, by the other.
Explanation IV.- Any matter which might and ought to have been made ground of defence or attack in such former suit shall be deemed to have been a matter directly and substantially in issue in such suit.
Explanation V.- Any relief claimed in the plaint, which is not expressly granted by the decree, shall, for the purposes of this section, be deemed to have been refused.
Explanation VI- Where persons litigate bona fide in respect of public right or of a private right claimed in common for themselves and others, all persons interested in such right shall, for the purposes of this section, be deemed to claim under the persons so litigating.
[Explanation VII.- The provisions of this section shall apply to a proceeding for the execution of a decree and reference in this section to any suit, issue or former suit shall be construed as references, respectively, to proceedings for the execution of the decree, question arising in such proceeding and a former proceeding for the execution of that decree.
Explanation VIII.-An issue heard and finally decided by a Court of limited jurisdiction, competent to decide such issue, shall operate as res judicata in as subsequent suit, notwithstanding that such Court of limited jurisdiction was not competent to try such subsequent suit or the suit in which such issue has been subsequently raised).

There is a distinction between issue estoppel and res judicata. Res judicata debars a court from exercising its jurisdiction to determine the lis if it has attained finality whereas the doctrine of issue estoppel is invoked against the party. If such issue is decided against him, he would be estopped from raising the same in the latter proceedings. The doctrine of res judicata creats a different kind of estoppel viz. estoppel by Accord; Bhanu Kumar Jain v. Archana Kumar, AIR 2005 SC 626.

Hope the above will clear your doubt about the difference between the two terms referred by  you.