Every user in Duplie has a "base" set of attributes (name, email, phone, etc.). There may be some organizations that need to store additional information on a user's profile. These are called Custom User Fields. You can create as many as you like. You can even make the fields required, to ensure you capture that information. Custom fields apply to ALL users in your instance.
To create a custom field you will need to be logged in as an admin. Then go to Users and then Custom User Fields. There you can create/edit the custom fields. You can setup custom fields to be checkboxes, dropdowns or just text boxes. You can even restrict them for admins only. When a new custom user field is added and set to be required for non admins, this will require all users to enter that information. As users login to your instance, they will be prompted to enter that missing field information. Custom fields for Admin Only will only be allowed to be viewed/updated by admins.