What is the distinction between an offer and an invitation to treat?

An offer is an expression of willingness to contract on specified terms, made with the intention that it is to become binding as soon as it is accepted by the person to whom it is addressed. An invitation to treat by comparison is inviting offers to be made, where the party whom the statement was made is then free to accept or reject. As a general rule, advertisements are invitations to treat, not offers and this was shown in the case of Partridge v Crittenden (1968); where it was held that the advertisement of the 'Bramblefinch cocks' was an invitation to treat and not an offer, as it was an expression of willingess to receive offers as the starting point of negotiations.