When creating or updating a campaign, you have the option to add 'tags' to the record. Tags allow you to search your account for campaigns that are related, and to create Report Builder reports using those tags. You can use any text you want in tags; just choose a system that works for you. 

For example, you might want to tag certain campaigns with 'video' to denote that they included a video. Or maybe certain pieces of content were intended as SEO cornerstone posts - you could tag them 'cornerstone'

To tag a campaign, just start typing in the tags field, or select an existing tag from the dropdown:

You can search for tagged campaigns on your calendar by prefixing your search term with a hashtag (#):

You can also build reports in Report Builder using tags: