Google Adwords Remarketing, or any remarketing campaign, works by putting cookies on the browsers of certain visitors to your site, enabling you to show custom ads to them after they leave. In the case of Google Remarketing, this means showing them your ads on the Google Content Network.
It seems like a great idea in principle – trying to recoup those marketing dollars you spent getting them there in the first place, by getting them back again to hopefully buy. But there are two points about remarketing campaigns in Google which can cause confusion for advertisers, and mislead them somewhat.
The first is regarding cookie length. Cookie length can be set up to as long as 180 days on your remarketing campaign. However, unless your product has a very long buying cycle or consideration time, your customer is likely to have already purchased an alternative by the 180 day mark. Effectively, what you are doing with the 180 day cookie then is setting up a content network campaign only for people who have already been to your site. So, you have to think about your cookie length very carefully, and make sure you understand what the difference is between remarketing to your visitors, and another campaign of content display ads.
The second thing, is the View Through Conversions. VTC are conversions where an impression has been made to the buyer of your remarketing ads, but not clicked on. However, what you might not realise is that the default setting in your conversion tracking is that view through conversions will also be counted when the buyer has clicked on an ad in another campaign. For example, a Search PPC ad. In my opinion, such conversions should not be attributed to both Search AND remarketing impressions, and I prefer to enable the deduplicating setting for conversions, which attributes it simply to the very last search click.