The Form I-20 is issued by a U.S. school to proves that you have been accepted and have met the met all admission requirements for your program and are eligible for "F-1" student immigration status. It also indicates how long you may remain in the United States.
An entry visa is placed in your passport at a U.S. embassy or consulate (outside the U.S.). An F-1 entry visa is required to ENTER the U.S. in F-1 student status (unless you are from Canada or Bermuda). The entry visa stamp may expire while you are inside the U.S. You will only need a new visa if you leave the U.S. after it expires and you wish to return.
The I-94 is your official arrival/departure record. It indicates at what port-of-entry and on what date you arrived into the U.S. It also indicates how you were admitted into the U.S. (for example, as a student or a tourist). It will also include an expiration date. For F-1/J-1 students/scholars and their dependents, this should be “D/S”, which means “Duration of Status.” This notation refers to the date you complete your studies or the end date on your Form I-20/Form DS-2019 (whichever is earlier) plus any grace period you may be eligible. If you enter the U.S. from a land border, you will get a paper Form I-94 stapled to your passport; otherwise, you can access the record electronically.