Google Tag Manager is Tag or Tracking Pixels Management System owned by Google to manage Java Script and HTML tags on websites.
There are three main parts of GTM or Google Tag Manager, which are as follows :-
Tags are pieces or snippets of Java Scripts or tracking codes or pixels from Google and third party tools.
Triggers are created to tell GTM how or when to fire a tracking code or pixel. Few examples of GTM are Google Analytics code, Adwords or Google ads remarketing code, Adwords or Google ads coversion tracking code, Facebook pixels, heatmap codes like Crazyegg or Hotjar.
Variables are additional information which GTM may require for the Tag or Trigger to work. Some example variables are Click Classes, Click Element, Click Element, Click Id, Click Target, Click Text, Click URL, Event, Form Classes, Form Elements etc.
What are Benefits of using GTM or Google Tag Manager
Major benefit of using GTM is that to place just a single pixel or script on website (GTM’s java script) and then add multiple tracking pixels in GTM itself rather than adding pixels every time to the website and have any kind of dependency on web developers.
Also, you can track basic events like Dowloads, PDF Downloads, links (outbound), or Clicks, Button Clicks, enhanced ecommerce tracking and promotion tracking, Add or Remove to Cart, shopping cart abandonment, form abandonment, video tracking , scroll tracking and much more.
Another major benefit of GTM is that you can test fire the pixel before actually taking it live on website and make sure it doesn’t break your website. Moreover this Preview can also be shared among your Colleagues too. This is known as Preview and Debug mode.
More Information regarding Preview and Debug mode here