The To:, Cc:, and Bcc: fields in the Compose window are powered by the Addressing Widget which enables you to search for contacts by either first name, last name, or even domain. For example, entering "ACME" will return all contacts within your address book with the domain "@acme.com".
Additionally, the Addressing Widget will pull in profile pictures it finds within the address book to make it easier to find and select the desired recipient.
Once an address have been selected, a contact "bubble" is created. You can use the pull-down menu to perform a set of actions, or drag the contact to a different addressing field.
To add more addressing fields such as Cc or Bcc, right-click to the left of the To: field.