A signature is a blog of text that appears at the bottom of every email you send. Salesforce signatures save you from having to manually sign every email sent from within Salesforce.com. Signatures can contain standard information such as your name, phone number and website address. An HTML signature can include additional features beyond plain text, such as a clickable link or even a picture. You can setup an HTML signature in Salesforce in just a few steps.
Log in to Salesforce.
Click your name in the upper-right corner and choose "Setup."
Choose "My Email Settings" under "Email" on the left-hand side.
Type or paste your HTML signature in the "Email Signature" box. Click "Save."