Urchin Tracking Module was an analytics package that Google acquired way back when.
UTM defines a set of query parameters that Google Analytics, and other web analytics platforms, use to capture and provide and information about a referring source (or 'channel') to a webpage. At the highest-level, the buckets are direct, organic, referrer, social, email, display, and paid search.
The three UTM parameters that are required to track 'campaigns' in a web analytics tool, such as Google Analytics:, are Utm_Source, Utm_Medium, and Utm_Campaign.
Example: http://stilldrinking.org/utms/utm-mangler.html?utm_medium=so...
If you clicked on the link above, it would inform stilldrinking.org's Google Analytics property that you, the visitor, came from Facebook and are part of the 'jan-15-hn-woop-woop' campaign.