Online advertising is something through which you can display your site or product ads on others site.PPC comes unders this .PPC means pay per click.One you display ads on other site.In case of PPC advertising you have to pay only per visiters or per click.Suppose per visiter site is taking .1$.Then some one click on your ads and after clicking come to your site.You have to pay .1$ to that publisher.
This is called PPC.Same way when you do advertising on your site , you also get money on per click when some click on ads displaying on your site.At that time you are publisher and client is advertiser.