Restrictive appositives are essential to the meaning of the sentence.

1. NON-RESTRICTIVE:

• Her husband, Fritz, is a nice guy.
  • We assume she has only one husband. Thus, commas are used.
    
    
• The firm chose Mary, vice president of public affairs, as its chief executive officer.
  • Because we have identified the person by name, her title is additional information. It can be set off by commas. In other words, we could take it out and the meaning would not change.
    
    
• The Grand Canyon, one of our nation's most popular tourist attractions, is breathtaking to behold.
  • Because we have identified the place by name, the rest is additional information. It can be set off by commas. In other words, we could take it out and the meaning would not change.
    
    
• Neil Armstrong, the first man who walked on the moon, is a native of Ohio.
  • Because we have identified the person by name, the additional information is not restricted to the sentence. It can be set off by commas. In other words, we could take it out and the meaning would not change.
    
    

2. RESTRICTIVE:

• Evan's friend John cheated on the test.
  • EVAN has more than one friend; therefore, no commas are used to set off JOHN. We need the name to know which friend we're talking about.
    
    
• We students are happy with good grades.
  • STUDENTS identify who WE [subj.] are. If we remove it, WE does not have the same meaning.
    
    
• She waited patiently for the famous author Stephen King.
  • STEPHEN KING identifies which famous author. There is no comma after AUTHOR because there are many famous authors.