Only what is offered can be accepted. This means that the offer must be accepted exactly as offered without conditions. If any new terms are suggested this is regarded as a counter offer which can be accepted or rejected.
There can be many offers and counter offers before there is an agreement. It is not important who makes the final offer, it is the acceptance of that offer that brings the negotiations to an end by establishing the terms and conditions of the contract.
Acceptance can be given verbally, in writing, or inferred by action which clearly indicates acceptance (performance of the contract). In any case, the acceptance must conform with the method prescribed by the offerer for it to be effective.