As Google says:
Why wait months for site code updates?
Google Tag Manager lets you launch new tags any time with a few clicks, so you never miss a measurement or marketing opportunity.
Urchin tracking modules - UTMs - are query parameters you add to your URL, which you then place in an advertisement, on Twitter, etc, so to know where people visiting your site are coming from.
Tag manager and UTMs are separate and do not depend on each other and their purposes are completely different.