However, as your data analysis requirements grow and you want to learn more about what your projects are doing and how you can increase the effectiveness of your content and campaigns, you will likely find that this general strategy is insufficient. A single tag that is inserted everywhere, can indeed collect a great deal of useful information, but you can learn more be inserting different tags in different pages within your project: Tags that report to Google when specific buttons are clicked, tags that group multiple related pages together, tags that report only if a video is played, etc. These multi-tag scenarios are not handled by GA, even though the data they collect are all aggregated by GA. The tags themselves are managed by Google Tag Manager (GTM).
GTM enables you to manage as many different GA tags as you need. To use GTM, you place Tag Manager code on your pages instead of GA tags. Then, using your GTM account, you define rules for GA behavior (reporting circumstances, timings, etc), and GTM automatically generates the appropriate GA tags and pours them on the page for you. When it’s time to change some or all GA tags, working in the GTM admin frees you further from direct contact with code. And this means you can react quickly to changing tag requirements, even if your project release cycle ordinarily moves more slowly (like being restricted to weekly or even monthly releases).
Finally, GTM can handle many different types of non-GA tags, including AdWords, LinkedIn, Salesforce DMP, Twitter, and more.