What is Google Tag Manager (GTM)?
Tag Manager is a tool by Google that centrally manages and publishes code used for displaying advertisements and recording visitor behavior on websites.
Before Google Tag Manager (GTM) existed, webmasters had to insert code on their pages for every tracking and advertising service they wanted to use. They also had to insert code into elements like links and buttons to track user behavior. However, with GTM the only code you have to add to your pages is GTM code.
GTM enables you to add tags. Tags can be as simple as Google Analytics tracking code or they can contain multiple variables and triggers that record or affect more complex visitor behavior. For example, you can add event tracking with GTM without having to add any onclick code to your page.
All tags are managed and hosted offsite using GTM. When you add or make changes to a tag, you publish it, and then Google automatically serves the updated code changes via the GTM code on your site.