A person is a contact, just like an organization is. Persons can be connected to Organizations
Persons are NOT users.
To create a person use the Add Person link on the homepage.
To edit a person, find it using the main search box and click the 'edit person' link to go to the edit person screen.
Add Person - add/edit screen
Creating new contacts exercise
How to add a login account (in version 4273 and before)