Source is where the contact came from, such as Facebook or your website. Medium is what the content was, such as a banner or sidebar ad. Campaign is what you're promoting or the name of your launch, such as "Spring Promo 2015."
For a Facebook Dark Post, you'd want to use something like this:
Medium: Dark Post
Campaign: Spring Promo July 2015
For an organic visit, you'd want something like this:
Source: Website Homepage
Medium: Sidebar Ad
Campaign: Organic Lead
UTM parameters are populated in the Contact Record only when a new contact is opting in, called "first attribution". The statistics in the Tracking Collection will update for both new and existing contacts, so using UTM is valuable even for existing Contacts.
If also using Google Analytics, Google recommends you include these three UTM parameters (Source, Medium and Campaign).