For years, this System Preferences pane was called Mail, Contacts & Calendars. It’s a single, handy, central place to tell OS X your user names and passwords for all kinds of Internet services: Gmail, Yahoo, Exchange, Twitter, Vimeo, Flickr, LinkedIn, and so on.
To get started entering your account names and passwords, click one of the big-name logos (Gmail, Yahoo, whatever)—or click the 
button below the list. The Mac is smart enough to feed the account information here into the right programs (Mail, Calendar, Contacts, and the Share buttons described on The Share Button ()).