Mail is anything that's delivered to your mail box or post office box — letters, bills, packages, magazines, or anything else that's sent through the postal service. Email is the internet's version of mail.
Mail carriers' bags are full of mail, and the system that allows this mail to be delivered all over the world is also called the mail. As a verb, to mail something is to send it via this system: when you write a letter, address and stamp it, and put it in a mailbox, you mail it. Mail is also the name for a type of light armor made of interlocking metal rings. If you read a novel about a knight wearing mail, he's wearing this type of armor — he's not covered in letters!