An Android contact contains information on a person with whom you are communicating generally by phone or email. A contact can contain name, photograph, phone numbers, email addresses, postal addresses, and also nicknames, relationships, events, notes ...
These contacts can be created by the user in a contact applications, or automatically by applications such as email applications, or recovered automatically if you chose accounts sync options and contacts are created on synchronized hardware.
The contacts are attached to the accounts you created on your mobile. The accounts are mainly mail accounts, but other account types are possible.
If there is no accounts created on your mobile, you may however create contacts with a null account.
Android examines all contacts created on the accounts, which we call elementary contacts, and if it finds several elementary contacts that appear to relate to the same person, it aggregates them into a contact. An Android contact may be by a single elementary contact, or be an aggregation of elementary contacts (usually of different accounts).