NationBuilder lets you tag any person in your database. A tag is a mark that you can assign to multiple people in order to keep track of them as a group. 

For example, if you have a survey page, you might be interested in keeping track of all of the people who answered a multiple choice question with a specific answer. The survey page allows you to tag everyone that chose that answer, so that you can view that list of people. You can also tag everyone on a certain list using a batch update, and manually enter a tag for a person in their control panel profile. 

Tags are useful when it comes to sorting through people in your database. If you want to use the map view function to see everyone that wants to attend a house party in a certain area, you can use tags to easily pull that list of everyone who checked yes to attenend a house party on the website.