Users vs Contacts

Hi all. I am new to NextCloud. I really don’t know what the difference is between users and contacts. Also, can you suggest some tutorials/documents about NextCloud? I have read Nextcloud User Manual The Nextcloud developers but it is not as detailed as However, is confusing and short.
Thank you so much.

A user has an account with a login and can use the system.

A contact is just a contact… like a contact in your phone.

Do I really need a phone number to create a contact? Also, I couldn’t find contact management in docs.

Here you go…

I don’t think so?