Measurement

GTM

Google Tag Manager

Google Tag Manager is a tag manager with two forms: a web container that runs in the browser and a server container that receives events and distributes them from your own infrastructure.

When it applies: It fits when marketing needs to change measurement without waiting for a release, and when a single place should show which tags exist and under what conditions they fire.
What it is confused with: Having GTM is not having governance. The container describes what fires, not whether it should. And the server container is not a better version of the web one: it is a separate piece, with its own cost and operations.
Architecture consequence: Every publish permission is permission to run code in a customer browser. Environments, change review, and who can publish to production are security decisions, not convenience ones.

Related concepts

TMS
sGTM
Data layer
CMP