Google Tag Manager Basic Concepts and Implementation Workflow
Google Tag Manager (GTM) is a free service by Google that is used to configure different analytics, conversion tracking and remarketing pixels (Universal Analytics tracking code, Facebook pixel, AdRoll pixel, etc.)
If GTM is not used all these codes and pixels should be added to the source code of the website. This approach makes it very difficult to implement the pixels (only technical team can do this), difficult to debug (tools are available only for few systems like facebook, twitter and bing) and difficult to update (source code changes are required).