Google Contacts is your address book. Store and organize contact information about the people you communicate with. Each contact can contain basic information like names, email addresses, and phone numbers but can also include extended information like physical address, employer, department, or job title.
The autocompletion item is turned on and all outgoing email address are gathered.
Google Training Documentation (At Google)
Google's Contact FAQ (At Google)
Different contact types
There are 2 different types of Google Contacts. Each is managed separately and contains similar, but not identical, information:
Google Apps Directory that is managed by IT Services and My Contacts. My Contacts is managed by each individual user.
To learn more about Contacts see Google's support page.