Welcome to Android Central! A factory reset irreversibly wipes the device--that's why it warns you at least once during the process. You can always sign back in to your Google account, and all of your Google data in the web (like your Gmail, Contacts, Calendar, etc.) will sync back with the device. You apps might also reinstall automatically, but this does not always happen for unclear reasons. But unless you were using a 3rd party launcher like Nova (which give you the option to back up your homescreen setup), you'll have to set up everything again by yourself.