Who pays for advertising?

My professor told me the TV advertising is not payed by the company who is advertising but by a third party. I didn't seen silly as everyone class seemed to know, so I didn't ask. But, what did professor mean as in third party?

Ask your professor-WHO is paying for a COCA-COLA ADS-MAY BE HI WILL SAY PEPSI ;)

Source(s):

TATZO.COM