A comma is used in sentences when you want to add some additional information related to the main clause without starting a new sentence. 

 

  They bought a new car, which was red in color, last month.


 Commas are also used to separate words in a list before using ‘and’.

 

  They bought eggs, milk, bread and jam at the supermarket.


 They are also used for the name or title of a person being directly addressed by the sentence speaker.

 

  Jane, will you pass me that book?
 

  I heard that, Professor Smith was the expert in this field. 


 Learn more - http://englishleap.com/punctuation/comma