what is lookup and how do i make the changes in the lookups ?

 on the Requisition form there is a standardrized lookup named the Requisition Type in which we have to select either Internal Requisition of Purhcase Requisition based on the requirement ... but it is customer requirement to make another option named as stevta requisition so that user can select though this option has all the option as same as that of Purhcase Requisition ..... CAN U TELL ME HOW DO I DO THAT ????