yep, employment contracts require some kind of offer and acceptance in the form of exchange between the parties. This is called consideration, which essentially means that in order for a contract to operate there has to be a principle of quid pro quo i.e. i pay you a salary in exchange for your labour. When most people think of contracts they think of them as being set upon paper and binding upon signing the dotted line; however this is not strictly the case, since a contract can exist on a simple verbal exchange and performance fulfilled upon what is agreed in the verbal exchange. i.e in the interview your prospective boss says yes you have the job; you agree and turn up Monday and start working = contract.
Therefore if you were to turn up on Monday and he was to say that he changed his mind over the weekend, you could sue him or more likely the company for breech of contract. Despite the fact that you have not signed a piece of paper headed “contract”.