UTM parameters
UTM parameters are labels added to the end of a link — utm_source, utm_medium, utm_campaign — that travel with the click and tell an analytics tool where the visitor came from.
They exist because a browser does not reliably say where a click originated. A link posted in an email or printed as a QR Code arrives with no referrer at all, so without a label the visit is filed as "direct" and the campaign that produced it is invisible.
The three that matter are source (where the link lived), medium (what kind of channel it was) and campaign (which push it belonged to). The other two, term and content, are for paid search keywords and for distinguishing two links in the same email.
The one rule that makes them worth using is consistency: "facebook", "Facebook" and "fb" are three different sources in every report that reads them, and nothing will merge them for you later.