Each bidding system ascribes a meaning to every possible call by each member of a partnership, and presents a codified language which allows the players to exchange information about their card holdings.
In serious online tournaments, the procedure is for the player making the call to self-alert it, but the explanation is visible only to the opponents.
Natural system(s) are the "lingua franca" of bridge players, with regional variations.
Players sometimes alter certain aspects of a system, adding their specific agreements or preferred conventions.
Examples are systems like Romex, Boring Club, Fantunes, and EHAA (Every Hand An Adventure).