In your 'contacts' list you will see a tab 'put in groups' along the top.
Clicking this allows you to create a group, name it, and move your contacts into that group.
Click Save.
From the same page, click the 'send mail' tab and you will see your group name in a box to the right. Clicking this will send your mail to all the addresses in the group.
You can have as many different groups as you like.