You must sign into a Google Account to use Gmail, Google Talk, Google Calendar, and other Google Applications; to download applications from the Play Store; to back up your settings to Google servers; and to take advantage of other Google services on your phone.
If you don’t have a Google Account, you’re prompted to create one.
If you don’t sign into a Google Account during setup, you are prompted to sign in or to create a Google Account the first time you start an application that requires one, such as Gmail or Play Store.
After you sign in, your contacts, Gmail messages, Calendar events, and other information from these applications and services on the web are synchronized with your phone.