You can add 3 types of custom fields:
Text: Any text field you want! (i.e. hometown, company, school, spouse, etc.)
Category: Use categories to sort your contacts. These are like Text Fields but across multiple contacts.
Date: Remember important dates (i.e. anniversaries, graduations, religious celebrations)
Once you open the contact detail panel from either the web dashboard or the Chrome extension, navigate to the "Details" tab to add a custom field.