An electronic "document" based on the International Telecommunications Union (ITU) X.509 (1988) standard consisting of a public/private key pair; their usage is governed by a Policy and a Practice Statement. They can be used for verification, encryption and digital signing. A digital certificate can also serve as an electronic notary seal (stamp). A certificate contains a digital signature, verified by another certificate - this creates a chain of certificates that ends with the 'root' certificate (which is self-signed); the owner of the root certificate is called the Root CA.
A Trust Policy can specify appropriate uses for a certificate: "should I trust this certificate for this action?". For example an S/MIME policy specifies that in order to be trusted to verify a digitally signed email, a certificate must contain an email address that matches the address of the sender of the email. This should also be part of an Assurance Framework.
The structure of a digital certificate is: Certificate
... a. Version
. . b. Serial Number
. . c. Algorithm ID
... d. Issuer
. . e. Validity
. ... i. Not Before
.. .. ii. Not After
... f. Subject
. . g. Subject Public Key Info
... . i. Public Key Algorithm
..... ii. Subject Public Key
. . h. Issuer Unique Identifier (Optional)
. . i. Subject Unique Identifier (Optional)
... j. Extensions (Optional)
Certificate Signature Algorithm
Certificate Signature.
Source: http://identityaccessman.blogspot.com/2006/08/identity-dictionary.html