A Kerberos ticket is issued by a trusted third party. It is an encrypted data structure that includes the service encryption key. In that sense it is similar to a public-key certificate. However, the ticket is not the key.
The following answers are incorrect:
public keys. Kerberos tickets are not shared out publicly, so they are not like a PKI public key. private keys. Although a Kerberos ticket is not shared publicly, it is not a private key. Private keys are associated with Asymmetric crypto system which is not used by Kerberos. Kerberos uses only the Symmetric crypto system. private key certificates. This is a detractor. There is no such thing as a private key certificate.