Source: New Horizons etips
You use a handwritten signature to verify that you’re the author or maker of a letter, check, or other paper document. You can use a digital signature to verify that you created or sent an electronic document. Digital signatures are based on public key cryptography, also called asymmetric cryptography because it uses two keys. The pair of keys is generated by a mathematical algorithm; they’re mathematically related, but the private key can’t be discovered by reverse engineering the public key. You distribute the public key freely to anyone who wants it, and keep the private key secret.
You can get a personal email certificate to digitally sign and encrypt your email messages free from the Thawte public certification authority at Personal E-mail Certificates