Your visa is a stamp in your passport that indicates that you are eligible to enter the United States in a specific immigration status (F-1, J-1, H-1b, etc). A U.S. border official will review your eligibility and authorize your requested immigration status by issuing the Form I-94 to show that you have been granted permission to be in the U.S. This small white card is your arrival/departure document and indicates your immigration status and the length of time you may remain inside the United States.
F and J visa holders will see the notation "D/S" on their Forms I-94. D/S stands for "duration of status" and refers back to the expiration date on your immigration document (I-20 or DS-2019). Employees with another visa status (such as H-1B or TN) will be given Forms I-94 with actual expiration dates by which time you are expected to leave the United States or apply for an extension of status.