In homage to its previous existence as Urchin Stats, Google Analytics supports URL tracking options using the prefix utm_, which stands for Urchin Traffic Monitor. There are five of them in total, and all you have to do is add them to any URL for your website that you publicise in some way. Typical publicity drives for websites are email, facebook and PPC (pay-per-click) advertising.
So, what are these tracking variables? In short, they are: utm_source; utm_medium; utm_term; utm_content; utm_campaign. You can add one or more of these to your website URL that you embed in a newsletter or include in an Adwords campaign, for example, and a typical URL would change from this:
It really is up to you what you put in these different parameters, but here are values that Google Analytics works out by default:
If you aren't sure how to piece all these bits together, the Google Analytics support site has a simple URL builder where you specify all of the bits and a fully-tracked URL is generated.