Google Tag Manager:
Google Tag Manager is a free tool that allows you manage and deploy marketing tags (snippets of code or tracking pixels) on your website (or mobile app) without having to modify the code.
Here's a very simple example of how GTM works. Information from one data source (your website) is shared with another data source (Analytics) through Google Tag Manager. GTM becomes very handy when you have lots of tags to manage because all of the code is stored in one place.A huge benefit of Tag Manager is that you, the marketer, can manage the code on your own. "No more developers needed.
"GTM is unfortunately misunderstood, overused, and abused. Although the idea of empowering marketers to easily do technical
stuff on websites was/is very appealing, the fallout of not fully understanding the technical implications of code insertion and
tagging can be detrimental to page structure and load time performance." – Angie Schottmuller, Conversion Optimizer