A signature is a bit of personalized text (like your contact information, favorite quote, etc.) that is automatically inserted at the bottom of every email you send.
To create a signature:
Select the "Settings" tab
and "General Settings" option in the left menu.
Type in your new signature text in the box labeled "Signature" and select the "Add following signature to outgoing emails" checkbox in the "Composing messages" section.
Click on the "Save settings" button.
If you don't want your signature to appear on a specific message, you can delete it manually before sending the message.