SSL uses a public key - private key system for its encrypting. The public key is used to encrypt the information. However, only the private key can be used to decrypt this information. This allows the public key to be known to all so that encrypted information may be sent to the server. The server can then use the private key, which is kept secret, to decrypt the information.
In this scenario, the certificate is the public key. A certificate signing authority such as Thawte or Verisign issues this public key. The public and private keys compose the digital key pair.