What is Google Tag Manager and how does it relate to Google Analytics?

Google Tag Manager (GTM) is a tag management system. You place one container snippet on the site, then deploy and modify all other tracking through the GTM interface without touching site code or waiting for a developer release.

The three building blocks:

• Tags — the code that fires, such as a GA4 event, a Google Ads conversion, or a Meta pixel.
• Triggers — when it fires: a pageview, a click matching a selector, a form submission, a scroll depth, or a custom event pushed to the data layer.
• Variables — the values passed, such as the page path, a clicked element's text, or a value read from the data layer.

The data layer is the important concept. It is a JavaScript object the site populates with structured information — order value, product details, user type — that GTM reads. Relying on scraping values from the DOM is fragile and breaks whenever the site's markup changes; a data layer is a deliberate contract with the developers.

The relationship to GA4: GTM is the delivery mechanism, GA4 is the destination. You can install GA4 directly, but GTM gives you version control, a preview mode for testing before publishing, rollback to previous versions, and one place managing every marketing tag rather than a page full of scripts.

Note: Always use Preview mode before publishing, and mention server-side tagging as the modern direction — it improves data quality against ad blockers and gives control over what is shared with third parties.