While an Interaction indicates meaningful communication that counts as a completed Follow-Up, a Note simply stores helpful information about the contact, such as timeframe for doing business with you, personal or professional interests, and important things to keep in mind. Sometimes you might consider communication as impersonal (like a flyer), and that can also be logged at a Note.
Notes are not included in the Total Contacted counter at the top of the contact record and do not count as completed Follow-Ups. If you want to log a phone call or meeting, or that you've sent a physical piece of mail, use Interactions.
To view all Notes for a contact, click Note in the header above your communication history with the contact.
Notes can be added from the contact sidebar or the contact profile. They can also be imported as part of a spreadsheet of contacts.
Via the contact sidebar
Click the contact's name on the Dashboard or Contacts tab to open the contact sidebar. Click "Add Note" under the "+" to write a Note.
On the contact profile
Search for the contact and open the contact's profile. Click on the Note section below the contact information and add your Note. Make sure to click Note first, so that you do not create an Interaction.