DoubleClick is Google's program for managing online advertising. Tags are a tool for analyzing advertising and learning which ads, campaigns or products generate the best sales. In 2010, Google upgraded its original DoubleClick program to offer new assets to publishers, as it refers to websites that host online advertising. Spotlight tags still exist as a legacy of the earlier program.
DoubleClick offers publishers and advertisers -- publishers are the sites that host the ads -- several services. Through DoubleClick, publishers control the time and location where ads appear, track the number of ads on their site to avoid over- or underbooking, forecast the site's future ad inventory and make the advertisements work for visitors reaching the website through mobile platforms such as smartphones or tablets. If you have to choose between ads offered by different online advertising networks, DoubleClick calculates which offers the best source of revenue.
Creatives are files or links that generate ads in DoubleClick. Advertisers embed tags on ad-carrying Web pages to track ads and monitor their performance. Each tag the advertiser inserts on a page tracks one specific datum -- click-throughs, or purchases, for instance -- so Google recommends inserting multiple tags to obtain maximum information. Placing tags shouldn't have any effect on the advertiser's site. DoubleClick used a software tool called Spotlight to track tags until March 2011, when Google replaced Spotlight with Floodlight.
Spotlight uses image tags, while Floodlight uses iFrame tags as the default tagging system. Both allow DoubleClick to track data, but Floodlight allows dynamic tagging. Dynamic tags allow third parties to collect and analyze the data as well as DoubleClick. If you want to use dynamic tags, you have to retag all your old Spotlight tags with new iFrame tags. If you leave your Spotlight tags in place, however, they'll keep working.
If you have active Spotlight tags, Google recommends you make regular checks on how well the tags have been implemented. Code for a Spotlight tag should be on the tagged page at the top, but not actually in the tag itself. If you plan to compare data from different tags, each tag should be implemented identically so the metrics match. You should see any line breaks in the Spotlight tag code.