It must have definite terms in order to be considered an offer (Gibson)
An invitation to treat is not an offer.
An advert is an invitation to treat (Partridge)
If an advertisement contains a clear indication that there is an offer, this becomes a unilateral contract, which only arises when the offeree completely performs a required act (Carlill v Carbolic Smoke Ball Co.)
Goods in a shop window (Fisher) or on a shelf (Boots) are an invitation to treat, as are lots in an auction (British Car Auctions)
A request for information is not an offer (Harvey).
An offer can be made by anyone or anything (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).
An offer can be revoked at any time, provided it is communicated (Routledge).
Saying 'no' or making a counter offer is rejection - the original offer cannot then be accepted (Hyde).
An offer can also end after a specified or reasonable lapse of time (Ramsgate Victoria Hotel) or after death.
An offer is a proposal or promise showing a party's willingness to contract.
An invitation to treat is an indication that a party is willing to negotiate.
Revocation must be before acceptance is communicated
may be carried out by a third party
promise to revoke is not binding without consideration