Add is a verb.
Meanings: include; attach; insert; affix; increase.
In mathematical terms add means to total or tally.
Ad is an abbreviation for the word Advertisement, which is a noun.
Meanings: commercial; public notice; poster; billboard; flyer.
Examples of Add:
1. The English Professor realized that giving excess examples would only add to the confusion.
2. The customer requested the shop owner add a satin bow on the lid of the gift box.
3. The Accounts Manager told the clerk to add up the figures in the audit book properly before submitting it.
Examples of Ad:
1. The Advertising Council of India summoned the Pharmacy company official for releasing an ad that gave misleading information about the new product.
2. Despite the huge amount spent on ads to promote the movie, the people stayed away from the theatre as the film did not feature any big stars.
3. An ad which is highlighted in bold is more likely to get people to see it than one which is does not have bold fonts.