This is a study of pembrolizumab (MK-3475, KEYTRUDA®) versus standard treatment (methotrexate, docetaxel or cetuximab) for the treatment of recurrent or metastatic head and neck squamous cell cancer (HNSCC). Participants will be randomly assigned to receive either pembrolizumab or Investigator's choice of standard treatment. The primary study hypothesis is that pembrolizumab treatment prolongs overall survival when compared to standard treatment.