Ever heard of the service called Gmail, I’m pretty sure that you have. Gmail is the email service provided by Google, launched back in 2004 as “Google Beta” with 1 GB space for the emails. Moving forward the Gmail service has been upgraded a lot of times, now the users have 15 GB of space.
When you sign up for a Gmail you are automatically opted in to every Google service like Google Drive, Google Docs, Picasa etc. All of these are Google’s services, you can use your main Google login to have access to all these websites.