>Like a certicate , digital certicate have a purpose that is To Establish TRUST. The pupose of using digital certificate varies depending on what kind of trust you need. Certificate can be used to verify identity of presentor. It can also be used to ensure that a object is intact or has been tampered.
Digital certificate can be used in following ways
To sign and encrypt mail
Single Sign On
For encrypted communication between client and server SSL protocol can be implemented. For example if you are owning a bank web site and want to encrypt communication between client and web server. To implement encrypted communication you need to hold a SSL Certificate. You can buy a certificate from a Certificate Authority or self generated Certicate can also be used. Whenever client access your bank site, certificate is produced before him.
Linux Security Cookbook