Postcard = Normal (Unencrypted) email
Letter in a sealed envelope = Secure (Encrypted) email
Postal service = internet backbone and the SMTP email delivery protocol
Mail box at your house = Gmail/Hotmail/Inbox
Public road next to your mailbox = Internet Service Provider (AT&T, etc)
Camera on the public road pointed at your mailbox = PRISM
Basically the NSA PRISM program is making a copy of every piece of mail you send as the postal service moves it from your mailbox into the postal system.
If you are not encrypting the email (like sealing a written letter in an envelope) before you put it in your mail box, anyone with access to the backbone (where SMTP protocol handles delivery between mail servers in plain text) can read it.
Anything you attach to that unsealed email (documents, pictures, etc) can also be read.