Contact Tags are here to make it easier for you to process your leads. You can tag each contact with a specific tag-name you like. Then, you can fastly search for all your leads with those tags from the Advanced Search feature.
This is How:
First, login to your ContactUs.com account and go to your Contacts tab.
Once in your Contacts, inside every lead, find the Tags field in the lower section of the left sidebar. Type in a tag and press "Enter" to type a new one. After every "Enter", the text will highlight and that's it, you've created a set of tags. You can place as much tags you want (or only one). They can always be useful when sorting and processing leads.