Server software runs on a high bandwidth box and communicates within protocol requirements (e.g. participating on a chat net or wiki mirror or other file mirror). Software requirements are constrained often by server software, and in turn, server software will also set hardware requirements.
Really this term is redundant, since server really means a piece of software. Using this term to mean a high bandwidth box dedicated only to one piece of such software is a common mistake, but it is a mistake. This is better called a host or (since that too can mean a person) server hardware - the hardware that runs "the server", i.e. the server software