Email programs will have some way of allowing you to read any of the mail you have gotten. It will look a lot like the editor you use to send mail, but you cannot usually write in a message you are receiving. There are usually a number of commands built into such a system which allow you some special flexibility:
Allows you to reply to the person who sent you that message. Usually asks you if you want to include the message you are replying to. It will then send you to the editor part of the program, automatically addressed to the person who sent you the mail you were reading. If you chose to include original message, you will see that message in the editor as well. This is especially handy if somebody asked you a number of questions in an email and you want to respond to them individually. You can edit a copy of their message and put the appropriate response after each question.
Allows you to send a message on to another person. If you received an email which somebody else should see, you can use a forward command to copy the mail to the new person. There are 'chain emails' which have been forwarded thousands of times across the Internet. They are just as annoying as the paper kind, but easier to delete.
Lets you save the message permanently. This is very useful when the document contains information you will need later. The document is saved as a plain text file you can manipulate with a text editor.
This command is very important. Once you become active with email, you will receive many messages. It is important to do a little housekeeping now and then so the number of messages in your account does not become completely out of hand.