You can tell by looking at the space you use. It is written at the bottom of the page. The bottom says the total space and the percentage you have used.
The tick box is gmail is used for marking. The marking could be used for many purpose. Deleting, moving, copying etc are some of the tasks.
Gmail can be used to get Emails mainly. It can also be used to get messages from hangouts. It is a complete networking service.
You can talk on chat using Gmail. Gmail has an instant messaging service in it too. It is called as Hangouts.
Folders are called as labels on Gmail. Gmail labels can be used to categorize mail. The mail can be moved to any labels.
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The address book is like a contract book in gmail. It can be used to store contacts in gmail. It is above the compose mail where Gmail is written and is a drop down.
You can create a Facebook account via Gmail. This links Gmail to connect with Facebook. The mails that you receive from Facebook are on this mail.
On Gmail you can make a little face that you prefer. Gmail supports a number of emoticons which are used to express different feelings.
Gmail is an account of Google where you manage your mails. It can also used for chatting with google friends. The Gmail account is a place for mailing services.
Yes, there is a Gmail App for windows phone. Gmail app is used to log in from phone. Multiple accounts can be logged in at once.
No Email ID is a verification for Gmail. It is used to verify if a user is a part of Gmail or not. The password and the ID form a unique combination.
Gmail App is a mobile version of Gmail for desktops. It can be used to manage mails as well. In gmail app, clicking on the icon will mark it and you can then delete it.