GTM: The Definition

What is GTM (Google Tag Manager)?

GTM is a tag management solution which acts as an intermediary between a website and 3rd party tracking tools.
You need to add the tracking codes to GTM and then configure the rules when they should fire.

GTM IS A TOOLBOX :D

Why using it?
1) With GTM we can test the tags to make sure they are triggered when we load a page or click a particular button.
2) We can change the tags without changing the website's source code.