Literally: Void means having no legal value and agreement means Arrangement, promise or contract made with somebody.  So void agreement means an agreement that has no legal value.

Traditionally: “An agreement not enforceable by law is said to be void”. [Sec 2(g)]

A void agreement has no legal effect. An agreement which does not satisfy the essential elements of contract is void. Void contract confers no rights on any person and creates no obligation.

Example of void agreement: An agreement made by a minor, agreement without consideration, certain agreements against public policy etc.

Agreement which become void:
An agreement, which was legal and enforceable when it was entered in to, may subsequently become void due to impossibility of performance, change of law or other reason. When it become void the agreement ceases to have legal effect.  

There are certain agreements, which are expressly declared to be void.
They are as follows:
(1)      Agreement by a minor or a person of unsound mind.[Sec(11)]
(2)      Agreement of which the consideration or object is unlawful[Sec(23)]
(3)      Agreement made under a bilateral  mistake of fact material to the agreement[Sec(20)]
(4)      Agreement of which the consideration or object is unlawful in part and the illegal  part can not be separated from the legal part [Sec(24)]
(5)      Agreement made. without consideration.[Sec(25)]
(6)      Agreement in restraint of marriage [Sec(26)]
(7)      Agreement in restrain of trade  [Sec(27)]
(8)      Agreement  in restrain of legal proceedings[Sec(28)]
(9)      Agreements the meaning of which is uncertain [Sec(29)]
(10)    Agreements by way of wager [Sec(30)]
(11)    Agreements contingent on impossible events [Sec(36)]
(12)    Agreements to do impossible acts [Sec(56)]

Some discussions on void agreement are as follows:

(1)    Agreement by a Minor Or a Person of Unsound Mind-
A person who has not completed his or her 18 years of age signifies as minor. Law acts as the guardian of minors and protects their rights, because their mental facilities are not mature- they do not possess the capacity of judge what is good and what is bad for them. Accordingly, where is a minor charged with obligations and the other contracting party seeks to enforce those obligations against the minor, the agreement is deemed as void.

A person who does not possess a sound mind or whose mental powers are not arranged or whose mental condition is not under his or her own control. Any agreement by person of unsound mind is absolutely void because he has no capacity to judge, what is good and what is bad for him.

(a)    A, 15 years old boy, made an agreement with B to give him Tk.1000. This is a void agreement.
(b)    A mentally disordered man made an agreement with X to marry her, but this is not a valid agreement.
(2) Agreement Made Without Consideration-
An agreement made without consideration is void, unless
1)    it is expressed in writing and registered under the law for the time being enforce for the registration of(documents), and is made on account of natural love and affection between parties standing in a near relation to each other; or unless.
2)    It is a promise to compensate, wholly or in part, a person who has already voluntarily done something for the promisor, or something which the promissory was legally compellable to do, or unless.
3)    It is a promise, made in writing and signed by the person to be charged therewith, or by his agent generally or specially authorized in the behalf, to pay wholly or in part a debt of which the creditor might have enforced payment but for the law for the limitation of suits.

In any of these cases, such an agreement is a contract.

Explanation 1–Nothing in this section shall affect the validity, as between the donor and donee, of any gift actually made.
Explanation 2- An agreement to which the consent of the promisor is freely given is not void merely because the consideration may be taken into account by the court in determining the question whether the consent of the promisor was freely given.

a)    A promises for no consideration, to give to B Rs. 1000; this is a void agreement.
b)    A, for natural, love and affection, promises to give his son, B Rs. 1000. A puts his promise to B into writing and registers it. This is a contract.
c)    A finds be B’s purse and gives it to him. B promises to give A Rs. 50. This is a contract.
d)    A supports B’s infant son. B promises to pay A’s expenses in so doing. This is a contract.  

(3) Agreements in Restraint of Marriage-
 Every individual enjoys the freedom to marry and so according to section 26 of the contract act “every agreement is restraint of the marriage of any person, other than a minor, is void.” The restraint may be general or partial but the agreement is void, and therefore, an agreement agreeing not to marry at all, or a certain person or, a class of persons, or for a fixed period, is void. However, an agreement restraint of the marriage of a minor is valid under the section.

It is interesting to note that a promise to marry a particular person does not imply any restraint of marriage and is, therefore, a valid contract.

This section enact that agreement in restraint of the marriage of any person, other than a minor is void. In the interest of the society, contracts for marriage are scrutinized with a close and vigilant suspicion of undue influence, fraud or imposition. The law presumes constrictive fraud, on grounds of public policy, in agreements respecting marriages since marriages of a suitable nature are of the deepest importance of the wellbeing of the society, as upon the equality and mutual affection much of their happiness, sound morality, and mutual confidence, hence every temptation of the exercise often undue influence, or a seductive interest in procuring a marriage is suppressed, for there is infinite danger that it may, under the guises of friendship, confidence, flattery or falsehood, accomplish the ruin of person especially females. So the law—
(a)    prevents improvident, ill-advised, and often fraudulent matches;
(b)    Avoid all such contracts as tend to the deceit and injury, or encourage artifices and improper attempts to control the exercise of free judgment;
(c)    Discountenances secret contracts made with prevents and guardians, whereby on a marriage, they to receive a benefits
(d)    Renders invalid certain agreements in restraint of marriage.

(a)    A agrees with B for good consideration that she will not marry C. It is a void agreement.
(b)    A agrees with B that she will marry him only; it is a valid contract of marriage.