If a phrase is meant to complement or introduce the main information in a sentence, it can be set off by a comma or pair of commas.

• Defining relative clauses
  
  In relative clauses do not use commas if the clause defines the antecedent and if omitting it would radically change the meaning of the sentence.
  
  

  Students who study here are very intelligent. Students who use the library are likely to be better prepared.

  
  
  The above clauses define the type of students referred to.
  
  In defining clauses that do not refer to people use either which or that.
  
  

  The research was completed with materials which were easily obtained. The research was completed with materials that were easily obtained.

  
  
  The above clauses define the type of materials referred to.

• Non-defining relative clauses
  
  Sentences with non-defining relative clauses need commas because the clause adds information to an otherwise complete sentence.
  
  

  Students at that university, who were all admitted with academic scholarships, have to work very hard.

  
  
  In non-defining clauses, only use which or who.
  
  

  The research, which was done over a period of three years, was conducted with easily obtained materials.

  
  
  In the following two sentences, which are both correct, the comma makes a difference. In the first, Dr Smith researches only those additives that pose a risk; in the second, the implication is that all food additives pose a risk.
  
  

  Dr Smith researches food additives which pose a risk to human health. Dr Smith researches food additives, which pose a risk to human health.