The principle of estoppel is an important concept of law of evidence. 

Under section 115 of Indian evidence act - When one person has, by his declaration, act or omission, intentionally caused or permitted another person to believe a thing to be true and to act upon such belief, neither he nor his representative shall be allowed, in any suit or proceeding between himself and such person or his representative, to deny the truth of that thing. 
Illustration A intentionally and falsely leads B to believe that certain land belongs to A, and thereby induces B to buy and pay for it. The land afterwards becomes the property of A, and A seeks to set aside the sale on the ground that, at the time of the sale, he had no title. He must not be allowed to prove his want of title. 

Essential Elements –
A.    A person misrepresents by his act, omission or declaration 
B.    Such misrepresent is intentionally caused to make a person believe a thing
C.    The other person believes such misrepresentation to be true
D.    Such act causes injury to the other person 
E.    Such person is unaware of actual situation 

Types of Estoppel -

 
1 Estoppel by Record - 
Estoppel by record is created by the decision of any competent court. When any court decides finally over a subject then it becomes conclusive and the parties, their representative, executor, administrator etc become bound to that decision. 

2 Estoppel by Deed 
When any person becomes bound to another person on the basis of a record regarding few facts, then neither that person nor any person claiming through him shall be allowed to deny it. or in other words When a party has entered into an engagement by a deed claiming certain fact, neither he nor his representatives can contest these facts later.

3 Estoppel by Conduct 
Estoppel by conduct is such estoppel which arises due to act, conduct or misrepresentation by any party. When any person causes another person to believe by his words or conduct or encourages them to believe and the other person acts upon that belief and causes change in their situation, then the first person is stopped from denying the truthiness of his statements made earlier. 

4 Equitable Estoppel 
Estoppel is a product of equity, therefore the Court will have to go by equities on both the sides to maintain a balance. A man may be stopped, not only from giving particular evidence but also depending upon that particular argument or contention that the rule of equity does not permit

5 Promissory Estoppel 
The history of promissory estoppels can be traced back to England, derived from the principle of equity. Once a party, by his words or conduct makes a legally binding promise to another party, if the other party has acted upon these words or conduct, the one who made the promise or gave assurance cannot revert back to his previous position.

Exceptions to law of estoppel 
1 Principle of estoppel does not apply to those matters where both parties have the knowledge of truthiness 
2 Principle of estoppel does not apply against statutes 
3 It does not apply to regulations 
4 It does not apply to ultra virus orders and decisions 
5 This does not apply to question of law 
6 It also does not apply to sovereign acts of the government 
7 Mere silence is not estoppel
8 The Principle does not apply against misrepresentation of minor. 

Difference between Estoppel and Resjudicata