Google Contacts is a way to 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 addresses, employer, department, or job title.
Google Contacts integrates with all Google Apps, such as Gmail, Docs, Calendar, through the autocomplete function. This allows users to send email, share documents, and schedule appointments without having to look up contact information in a separate tab or window. Users can also synchronize contacts between Google Apps and a mobile device to see contacts outside of a browser.
Basics: Contacts
Email, find, and manage your contacts.
Google also has detailed help for common issues on their Contacts FAQs wesbite.