Homophones: ad vs add

Here is an example of homophones that are confused very often. You can visit out English Grammar section to find the general explanation for homophones.

Here is the meaning of these words according to Dictionary.com

• Ad
  noun
  
  1. advertisement.
  2. advertising: an ad agency.

• Add
  
  verb (used with object)
  1. to unite or join so as to increase the number, quantity, size, or importance: to add two cups of sugar; to add a postscript to her letter; to add insult to injury.
  2. to find the sum of (often followed by up ): Add this column of figures. Add up the grocery bills.
  3. to say or write further.
  4. to include (usually followed by in ): Don't forget to add in the tip.
  
  verb (used without object)
  5. to perform the arithmetic operation of addition: children learning to add and subtract.
  6. to be or serve as an addition (usually followed by to ): His illness added to the family's troubles.
  
  noun
  7.Journalism. copy added to a completed story.
  
  Verb phrases
  8. add up to, to signify; indicate: The evidence adds up to a case of murder.
  
  Idioms
  9. add up,
  
  1. to make the desired, expected, or correct total: These figures don't add up right.
     
  2. to seem reasonable or consistent; be in harmony or accord: Some aspects of the story didn't add up.

Source: Dictionary.com