A contract is an agreement giving rise to obligations which are enforced or recognised by law. As briefly mentioned above, in common law, there are 3 basic essentials to the creation of a contract; offer and acceptance create an agreement; consideration and contractual intention. If we consider the first requisite of a contract we will see that the parties should have first reached an agreement. Generally speaking, an agreement is reached when one party makes an offer, which is accepted by another party. Let us break down even more each of the elements that form a contract starting with:
An offer is a manifestation or communication of readiness to contract on stipulated terms, made with the intention that it is to be binding once accepted by the person to whom it is addressed. There must be an objective appearance or exhibition of intent by the offeror to be bound by the offer if accepted by the other party. Therefore, the offeror will be bound if his words or conduct are such as to persuade a reasonable third party observer to believe that he intends to be bound, even if in fact he has no such intention. An offer can be addressed to a single person, to a particular group of persons, or to the whole world. An offer may be made expressly (by words) or by conduct.
An offer must be differentiated from an invitation to treat, by which a person does not make an offer but invites another party to do so. So how can you tell if a statement is an offer or an invitation to treat? This will depend predominantly on the intention with which it is made. An invitation to treat is not made with the intention that it is to be binding as soon as the person to whom it is addressed communicates his consent to its terms. Invitations to treat include advertisements or displays of goods on a shelf in a self-service store and are usually not a contract. Partridge v Crittenden  1 WLR 1204, Pharmaceutical Society of Great Britain v Boots Cash Chemist (Southern) Ltd  1 QB 410.
However, the super famous case of Carlill v Carbolic Smoke Ball Company  EWCA Civ 1 is different and therefore relevant here. This English contract law decision by the Court of Appeal, which held an advertisement containing certain terms to get a reward constituted a binding unilateral offer that could be accepted by anyone who performed its terms. It is notable for its curious subject matter and how the influential judges, (particularly Lindley LJ and Bowen LJ), developed the law in inventive ways.
The details are as follows: A medical firm advertised that its medical preparation or new drug, called “The Carbolic Smoke Ball” would cure flu, and if it did not, buyers would receive £100. This meant that any person who contracted the influenza after having used one of their smoke balls in a specified manner and for a specified period would receive £100 from the medical firm. Mrs Carlil on the faith of the advertisement bought one of the balls, and used it in the manner and for the period specified, but nevertheless contracted the influenza. Mrs Carlil approached the medical firm for her £100 but the company refused leaving Mrs Carlil no choice but to sue. When sued, Carbolic argued the advert was not to be taken as a legally binding offer as there was no contract because it lacked the typical offer and acceptance; it was merely an invitation to treat, a mere puff or gimmick. However, the Court of Appeal held that the advertisement was an offer of a unilateral contract between the Carbolic Smoke Ball Company and anyone who satisfies the conditions set out in the advertisement. Once Mrs Carlill had satisfied the conditions she was entitled to enforcement of the contract; the notification of implementation of the conditions formed part of the acceptance. An intention to be bound could be inferred from the statement that the advertisers had deposited £1,000 in their bank “showing our sincerity”.
An acceptance is a conclusive and definite expression of agreement to the terms of an offer. Once again, there must be an objective appearance or exhibition of an intention by the recipient of the offer to be bound by its terms. An offer must be accepted in accordance with its specific terms if it is to form an agreement. It must accurately and faithfully match the offer and ALL terms must be accepted. The legal effect of acceptance only comes in to existence when it is communicated to the offeror. Communication of acceptance is an important element and without communication to the offeror, the offeror will be bound without knowing that his offer had been accepted and this could cause all manner of hardship.
Can an offer be accepted by conduct or are just words acceptable? An offer may be accepted by conduct for example, an offer to buy goods can be accepted by sending the goods to the offeror.
Are there other ways that an offer can be accepted? Yes. With regards to postal acceptance, the general rule is that acceptance takes effect when the letter of acceptance is posted and this includes situations when the letter becomes delayed, lost and even destroyed. However, there is one instance that the postal rule will not apply and that is if it is excluded by the express terms of the offer, Adams v Lindsell  1 B & Ald 681 and Henthorn v Fraser  2 Ch 27.
With regards to acceptance via an instantaneous medium such as email, it will take effect at the time and place of receipt, Entores v Miles Far East Corp  2 QB 327.
What is the situation where acceptance needs to be communicated in a specified way? This means if communication of acceptance is communicated in another way, it will not be accepted; it can generally be accepted only in that specified way.
If a communication attempts to vary the terms of an offer, it will fail to take effect as an acceptance. The situation posed here is rather a counter-offer. Hyde v Wrency  3 Beav 334. The original offeror has a choice whether to accept or reject the offer. It is not compulsory to accept a counter offer. A good example is when the offeror offers to trade on its standard terms and the offeree purports to accept, but on its own standard terms, this represents a counter-offer. Here we see that making a counteroffer amounts to a rejection of the original offer which cannot consequently be restored or accepted unless the parties agree.
Counter-offers are not the same as a request for further information. It is important to distinguish the two because a mere request for further information is regarding the original offer and not a new or counter-offer.
An offer may be withdrawn at any time before its acceptance; however the withdrawal must be communicated to the offeree; if it is not communicated, the revocation is ineffective. A withdrawal need not be communicated by the offeror personally because it can be made by a reliable third party.
Lastly, an offeror cannot stipulate that the offeree’s silence amounts to acceptance. Acceptance must be communicated in one of the forms above.
Once an offer has been accepted, the parties have an agreement. That is the basis for a contract, but is not sufficient in itself to create legal obligations.
In common law, in order for a promise to be binding, there needs to be consideration unless it is made as a deed.
What is consideration? Consideration is “something of value” which is provided for a promise and is obligatory in order to make the promise enforceable as a contract. If only one party offers consideration in contract, the agreement is not legally a binding contract. For example, payment by a buyer is consideration for the seller’s promise to deliver goods, and delivery of goods is consideration for the buyer’s promise to pay.
Consideration must be sufficient, but need not be adequate. What does this statement mean? This means that consideration must have some value, but this value does not need to match the return of the agreement. For example, A wants to buy B’s brand new bag. The bag is worth £100 but A will only pay £40 for it. The consideration is £40 which has some value but this value does not match the price of the bag. Usually, a promise has no contractual force unless some value has been given for it.
Law courts do not usually get involved with whether there is any economic equivalence between the value of the consideration given and the value of any goods or services received. You may ask why? This is because they do not normally interfere with the bargain made between the parties. Consequently, nominal consideration is sufficient; however consideration must not be from the past. The consideration for a particular promise must be given in return for that promise. The promisee must provide the consideration.
Now that we know what consideration is, what does not constitute consideration? Consideration is not an informal gratuitous promise because this does not amount to a contract.
Consideration is vital with regards to enforcing a contract. Customarily, a person to whom a promise was made can enforce it only if the person themselves provided the consideration for it. In fact, a person does not have a leg to stand on if the consideration moved from a third party. Let us look at an example: if A promises B to pay £10,000 to B if C will paint A’s house and C does so, B cannot enforce A’s promise, (unless B had procured or undertaken to procure C to do the work).
However, where the conditions of the Contracts (Rights of Third Parties) Act 1999 are met, a third party may be able to compel rights created in his favour by a contract which he was not a party to, and the courts are also adopting a more adaptable position under the common law here, which is progressive. While consideration must move from the promisee, it need not move to the promisor.
Consideration may be satisfied where the promisee suffers some detriment at the promisor’s request but confers no corresponding benefit on the promisor.
Exceptions to the rule – an example of such is where certain terms of a contract are void either by statute or where common law holds the terms to be so unreasonable that they cannot be enforced and/or are varied by the courts. If consideration is given by the promise in this circumstance, the contract will still not be enforced.
Another exception is where consideration may move from the promisee without moving to the promisor where the promisee, at the promisor’s request, confers a benefit on a third party. In situations where goods are bought with a credit card, the issuer makes a promise to the supplier that s/he will be paid. The supplier provides consideration for this by providing goods to the customer.