To ensure that your ad campaign is performing as well as possible, you should really be gathering conversion data. This means tracking whether your ad is actually leading to the action you want it to, whether that be selling an item, gathering email addresses, or just sending a user to a certain page. How to implement such tracking differs depending on who you are advertising with, and what you are advertising, however some sort of attribution system is a great idea whenever possible.
If you want to track whether an ad has sent user to a specific page, the easiest way to do this is to simply have a version of a page which is only used for that ad (as in a duplicate with a slightly different URL). There are other ways to do this, but having a duplicate page is probably the easiest and most accurate way – simply have your ad link to it and you can see how many users your ads are driving to your site. Note: This will be slightly less than the amount of clicks recorded on your ad as some users will drop off before your landing page loads.
If you want to track whether an ad has led to a sale, you can also have a specific sale area for an ad if you want, although this becomes somewhat inefficient as users tend to look around sites before completing their purchase. There is also something called a “post-impression conversion” which is where a user sees your ad, but doesn’t click on it, but goes to your site independently and makes a purchase because of the ad. To attribute such purchases to your ads is a difficult proposition, however most Ad Networks have a solution – simply ad a piece of code to the ad and to your site at the point of purchase. The Ad Networks system will then keep track of everything else.