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You can read here all about Google Tag Manager, https://support.google.com/tagmanager/answer/2574305?hl=ena

Google Tag Manager makes it easy for marketers to add and update website tags including analytics, remarketing, and more as explained by google.

Google Tag Manager is a free tool that makes it easy for marketers to add and update website meta tags without needing to edit your website code.

How does Google Tag Manager work?

Google Tag Manager is a code management platform that fires all of your other tags according to triggers you specify in the Google Tag Manager interface. Google Tag Manager is platform neutral and handles 3rd party tags with ease. Once the container snippet is deployed on your site or mobile app, little to no IT or web developer involvement should be necessary to deploy new tags or edit existing tags.

At a high level, Google Tag Manager works like this: You place a small snippet of code on your website or mobile app. This code connects your app to the Tag Manager servers. You then use a web based user interface to install tracking codes on your site or mobile app, such as Google Analytics tags. You can configure triggers to fire when an event occurs. An event could be a page load, a form submission, or a tap on a screen. When the trigger fires the tag, the tag's code executes.

Here's an example sequence: Tag Manager detects a page load event. The tag is set to fire when the page loads, and fires a Google Analytics tag. The Google Analytics tag then sends its data back to the Google Analytics servers.

The advantage of this is that once Tag Manager is properly set up on your website or mobile app, you usually will have to write no additional code to deploy or modify tracking codes for your marketing or analytics campaigns. It's all done through Tag Manager's web interface.

Google Tag Manager doesn't reduce the number of tags on your site or app, but it does simplify the task of managing them. For websites, Google Tag Manager executes asynchronously and can be configured to fire tags only when they are needed, helping your pages to load more quickly.

Google Tag Manager keeps track of which tags should fire and the triggers that have been set up to cause those tags to fire. Each time Google Tag Manager is engaged, the most up-to-date tag configuration is sent to the visitor with a set of tags and triggers. As the user interacts with your content, triggers are evaluated based on the events you have specified, and tags will fire accordingly.

Use Google Tag Manager to manage tags (such as tracking and marketing optimization JavaScript tags) on your site

Google Tag Manager is a powerful, free tool that enables marketers to control digital marketing data by using code snippets on any website. It can be used for conversion tracking, website analytics, retargeting, and many more tracking purposes.

Google Tag Manager (GTM) is a tool that gives users a way to add code to a website without having to log in to the backend and manually add to the source code every time. This takes web development and coding skills out of the equation, and is an easy way for anyone to add and update tracking codes all in one place in a consistent fashion.