The Google tag (gtag.js) is a single tag you can add to your website to use a variety of Google products and services. Instead of managing multiple tags for different Google product accounts, you can use the Google tag across your entire website and connect the tag to multiple destinations.
The Google tag was previously referred to as the global site tag.
If you set up Analytics on your site after August 2017, your site is probably tagged with gtag.js. If you set up Analytics on your site before August 2017, your site is probably tagged with analytics.js.