Adding a contact tag is easy. On any contact, scroll to find the "add tag" button. Select/Enter tags and save:
Now you can search for any tag from the contacts page to get all contacts tagged with it:
To see a breakdown of performance and conversion for your tags, you can use the tags view on the contacts page:
Tags view shows how many contacts were added for each tag for a given time range and how many of those turned into installations:
You can also go to the Analytics tab and select the "Contact Tags" segment from the segments dropdown to see a list of all tags for any date range:
Lead Form Tags
If you use lead forms, you can go to a lead form's settings and set some tags. The tags you select on lead forms will be carried over to all leads submitted with that form: