What is the difference between Contacts, Shared Contacts and Profiles?

[Julian (Google)]

A new item in our FAQ: http://code.google.com/apis/apps/faq.html#sharedcontactsexplained

In Google Apps there are 3 different kind of contacts and each kind
has a different API:

1. Contacts: The Contacts Data API allows client applications to view
and update user's contacts, these contacts are unique to the user and
not shared with anyone else. Your client application can use the
Contacts Data API to create new contacts, edit or delete existing
contacts, and query for contacts that match particular criteria. For
more information visit: http://code.google.com/apis/contacts/docs/3.0/developers_guide.html

2. Profiles: The Profiles Data API allows client applications to
retrieve and update profile information for users in a Google Apps
domain, the profile information is shared with all users in the domain
if contact sharing is enabled. Also, the profile information can only
be modified by the domain administrators. Every user has a profile
even if it is never used and thus a profile cannot be inserted or
deleted. It is important to mention that profiles includes only users
of the domain, it does not include nicknames or groups (email lists).
For more information visit: http://code.google.com/apis/apps/profiles/developers_guide_protocol.html

3. Shared Contacts: The Shared Contacts API allows client applications
to create, retrieve, delete and update contacts that are shared to all
users in a Google Apps domain, except for the domain's users (e.g.
us...@domain.com). Shared contacts are visible to all users of the
domain if contact sharing is enabled. It is important to mention that
shared contacts should be used to add contact information of the
domain's nicknames and groups (email lists). For more information
visit: http://code.google.com/apis/apps/shared_contacts/gdata_shared_contacts_api_reference.html

Regards,
--Julian

[Kevin Marks]

then there's the Portable Contacts API:

http://code.google.com/apis/contacts/docs/poco/1.0/developers_guide.html