Sender Signatures are used to avoid spam.
If you’re using a service like Outlook or Gmail you won’t even be aware of this.
However, if you send an email programmatically then you need to say who the Sender is. You usually do that using a Reply-To header. Unfortunately spammers have being doing this forever so, to avoid an email being labelled as spam, you’ll need to sign your email.
There are two main methods of creating Sender Signatures: DKIM and SPF.
DKIM (DomainKeys Identified Mail) has emerged from IETF (one of the Internet governing bodies) specifications and uses a signer’s public key which is published in the DNS. More here: http://en.wikipedia.org/wiki/DomainKeys_Identified_Mail
SPF (Sender Policy Framework) adds the list of authorized sending hosts for a domain in a TXT record. More here: http://en.wikipedia.org/wiki/Sender_Policy_Framework
For other methods see: http://en.wikipedia.org/wiki/Email_authentication