And here's some more detail
1An offer
An offer by the party offering something in exchange for something such as a product or service. The written agreement will contain proposed terms. Typically, a quotation from a product or service supplier is an offer.
Circumstances to watch out for
offer was rejected
If an offer was rejected by the party it was directed to, then it ceases to be an offer.
offer was withdrawn
If the party who made the offer then withdraws is, then it ceases to be an offer provided that it was not already accepted by the party the offer was directed to.
expiry of offer
If the offer expires, then it ceases to be an offer. This usually will be a written term in the agreement.
there was a counter-offer
If an offer was not accepted, but the person who the offer was directed to made a counter-offer, then the orginal offer ceases to be an offer.
uncertain or incomplete of terms
If important or "essential" terms in an agreement are uncertain, incomplete, vague or meaningless; and if additional evidence cannot establish what the term was meant to mean, then there cannot be a valid offer.
commercial offers over $40,000
The Australian Consumer Law will imply terms (e.g. guarantees that products are fit for their intended purpose) into written agreements unless it is a business related transaction where the value of the product or service is over $40,000.
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2Acceptance
Acceptance by the party who the offer is directed to. An acceptance shows a willingness by that party to accept the offer without further negotiation, and be bound by the proposed terms in the offer.
Some written agreements, such as agreements for the purchase of a property or a mortgage, are more “formal” than others which means to be validly accepted they must be signed. With less formal agreements, such as insurance agreements or a mobile phone contract, it can be sufficient to accept over the phone, online (e.g. by ticking a check box) or by email.
Circumstances to watch out for
acceptance must be communicated
An offer cannot be considered accepted if it was not communicated to the party that made the offer. This must be some direct means of communication which may be required formally such as by signing the agreement, or informally such as an email acceptance.
acceptance of partly written and partly verbal terms
Be aware that unless one of the terms in a written agreement is an "entire contracts" or "complete agreement" term (this means that only the terms in the written agreement are enforceable), then an agreement may be partly verbal and partly written. The effect is that any pre-contractual promises discussed verbally and agreed to, or previous regular dealings parties may have had with each other that are not present in the written terms, may form part of the overall agreement. This is known as a "collateral" contract as it sits side by side to the main written contract. To be a collateral contract, the verbal promise must be such that it motivated the party that the offer was directed to to enter into the main written contract.
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3Consideration
Something of value exchanged for what was offered. It could be something of value given or promised such as money, a service or even a peppercorn.
Circumstances to watch out for
adequacy of consideration
Any consideration can be good consideration, therefore it need not be adequate consideration. This means that an agreement will be valid even if one party to the agreement does a bad deal and offers greater value than what is exchanged in return.
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4Intention to create legal relations
This is to ensure that both parties wanted to be legally bound by the agreement.
Circumstances to watch out for
where no consideration provided
If there was no consideration exchanged then it may stand to reason that the parties did not intend to be legally bound by the agreement.
non-binding term
If one of the terms of the agreement states that the agreement is "non-binding" then it stands that the parties did not intend to be bound by the agreement.
agreement is unsigned
Presence of signatures is a strong if not conclusive indication that parties intended to be bound by an agreement. If an agreement is unsigned, however the parties start "performing" or fulfilling their obligations under the terms of an agreement, then it may be inferred that the parties intended to be legally bound.
commercial agreements
In business to business or commercial agreements, it is usually presumed that the parties are intended to be legally bound.