Syncing contacts with google account is the one of the easiest way to backup and transfer contacts between smartphones.  Like Android smartphones, even Windows smartphones too provide the option to sync gmail contacts. This method of syncing contacts works with all the Windows Phone OS based smartphones such as Lumia 920,820, HTC 8S, 8X etc.
Syncing gmail account with windows smartphone

Steps to add  and Sync gmail account with Windows smartphones

Step1: First go to settings on your windows smartphone which is available in the list of applications when swipe left or right on your home screen
Step2:  In "Setting', choose 'email+accounts'.  Tap the option to open
Step 3:  This will show you list of accounts which you have already added. Normally when you are doing first time, only microsoft account will be there. . Choose 'add an account' option.
Step 4: You will get a list of accounts you can add it to your phone. Choose Google.
Step5: The above step takes you to a page which ask for the login credentials of your gmail account. Enter your user name and password  and tap on to 'Next'. You need to enter your full email address. For example, abc@gmail.com.
Step 6: You will get a screen which asks you to choose which content should be synced. Select, Email, Contacts and calender. This will import your contacts, emails and calender to your smartphone.
Step 7: Tap on Sign in. By automatically you will taken back to the email+account settings screen. You will see the progress of syncing activity.
Step 8: You are done. Your smartphone will now sync with google account.

Adding and syncing with google is one of the easiest and best available option. Because, even if we lose our smartphones, we will not lose the contacts. Once sync is complete, you will get full contact details such as phone numbers, email address and other details which are available.
Similarly, you can add and sync account with Facebook, Yahoo mail, Twitter, Linkedin etc.