Google Groups help you in sending mails to particular set of you people you repeatedly are in touch with.
Rather than entering the user names manually in 'To' ,'CC' and 'Bcc' fields you can create a group of the peoples you want to send mails.

Here are the steps to create groups in Google Apps

Step 1: Login into your Admin Console of Google Apps.

Step 2: You will find an Groups icon on the dashboard or in the more controls option.

Step 3; click on it and you will get a window in which you have to enter the fields like Group name, Group email id which you want or which would be easier for you to recognize.

Step 4: Add members by entering their email addresses.

Step 5: After you are done with your Group creation you have to just enter the Group email id in 'To' field and the mails will get distributed to all the members in the group.