Just as your signature on a document attests to your understanding and acceptance of the content you have signed, a digital signature can be applied to a document in the same way. A digital certificate is akin to a signature card at the bank: you sign the card and the bank uses it to verify the signature on a check.

Your signature in the digital world is a unique mathematical equation. To be valid in commerce, a third party must keep a copy of your Public Key to verify your signature. There are several companies, such as Entrust, RSA and VeriSign, that issue and manage digital signatures.

There are additional ways to apply security, of course. For instance, the US Navy is using a digital ID that also has a biometric component, and analyses a signature written on a signature pad.

Certificate Security is more powerful than password security, and less likely to be easily broken. Certificate Security is not password dependent, although you use a password associated with the digital signature when applying it to a document.

A document can be certified or digitally signed by application of a digital signature which remains valid unless the document is modified in ways other than those specified at the time of signing.