Categories are groups of contacts. Categories are useful for small mailing lists, sorting, or however you wish to group your contacts. You can send email to categories of contacts by simply typing the name of the category in the To, CC, or BCC field in an email and auto-fill will enter in the addresses for you.
By default, All Contacts are displayed. Click there to create new or access categories. Categories are located on the top right of the screen.
Select a category or enter a name for a new category and click the + button to add.
After creating a category, you can add contacts to categories by selecting the contact(s) and clicking "Add to Category".
Now, when you select the category, or categories, you wish to view from the top right, only the contacts in that category, or those categories, will display.