A digital signature is a secure way of signing electronic documents such as an email, Microsoft Office document...
or PDF. These signatures can be legally binding, just as a written signature is. Instead of seeing a written signature on an email, the recipient will see a series of numbers and letters that can only be generated by the publisher.
When a user creates a document, he or she signs it with a unique digital signature and sends it to the recipient. If the publisher's signature is managed by a certificate authority (CA), such as those listed above, the recipient will trust the CA to confirm the identity of the publisher. This authenticates the message and provides nonrepudiation.
Any person, firm, company, agency, etc. can get a digital signature. There are several companies, known as certificate authorities (CA) that manage digital signatures, such as EMC Corp.'s RSA security division and Digi-Sign Ltd.