All contacts in Android work in the context of an account. What is an account? Well, for example, if you have your e-mail through Google, you are said to have an account with Google. If you set up yourself as a user of Facebook, you are said to have an account with Facebook.
Even though you use only the e-mail service with Google, the same login and password could be used to access other Google services. Your Google e-mail account is not limited to just e-mail. However, some accounts are restricted to just one type of service such as a Post Office Protocol (POP) e-mail account. On your mobile device, you may be able to register with a variety of these account-based services.
You will be able to set up some of these accounts ...