A contract is concluded when the contracting parties agree on its essentials, i.e. when there are two identical manifestations of will and intent to enter into a contract. Contracts are not usually bound by any specific form (unless a written or other form is required by law for a particular type of contract, e.g. a licensing agreement granting an exclusive licence must be made in writing).
As is often the case in practice, a contract may be concluded verbally, for example by phone or handshake. No additional written confirmation is required for such contract to be effective. However, in case a dispute arises during the term of a verbally concluded contract (for example if deadlines are not met, payments are not received in time or not at all, or there are disagreements over the performance of the contract, etc.), it is very useful to have the contract in writing, because anyone who refers to a contractual obligation of the other party also has to be able to prove the existence of such obligation.