offer and acceptance

• An offer is the starting point of a contract. It must have definite terms in order to be considered an offer (Gibson v Manchester City Council).
• An invitation to treat is not an offer. An advert is an invitation to treat (Partridge v Crittenden). Exceptionally, if an advertisement contains a clear indication that there is an offer, this becomes a unilateral contract,  which arises only when the offeree completely performs a required act, (Carlill v Carbolic Smoke Ball Co.)
• Goods in a shop window (Fisher v Bell) or on a shop shelf (Pharmaceutical society v Boots) are an invitation to treat as are lots in an auction (British Car Auctions V Wright.)
• A request for information is not an offer (Harvey V Facey.) An offer can be made by anyone or anything (Thornton V Shoe Lane Parking.) An offer will come into existence when it is communicated to the offeree. It is required that the offeree knows about the offer (Taylor v Laird.) It is also crucial that the exact timing of the offer being made is known (Stevenson v McLean.)
• An offer can be revoked at any time, providing it is communicated (Routledge V Grant.) Saying no or making a counter offer is rejection. The original offer cannot then be accepted (Hyde V Wrench.) An offer can also end after a specified or reasonable lapse of time (Ramsgate Victoria Hotel v Montefiore.) Death also ends an offer. 
• Acceptance is the final and unconditional agreement to all the terms of an offer. Acceptance cannot be silence (Felthouse v Bindley.) An offeror can require a specific form be taken to convey acceptance, though these requirements can be waived (Yates v Pulleyn). Auto generation in an email can be the same as a signature on paper (Neocleous v Rees ).
• The postal rule states that where post is the usual or expected form of communication and has been properly addressed and stamped then acceptance is at the point of posting (Wells v Devani.) In any other circumstances, acceptance occurs when the offeror is aware of the acceptance (Entores v Miles Far East.) In Brinkibon Ltd v Stahag Stahl electronic acceptance occured when the message was opened.