When configuring the ESA under Mail Policy > Sending Profile, the two "Domain Key Type:" are DKIM and Domain Keys.
DKIM (DomainKeys Identified Mail): This is a method for associating a domain name with an email message, thereby allowing a person, role, or organization to claim some responsibility for the message. It is widely used and is the successor to Domain Keys.
Domain Keys: This is an older standard used for email authentication that works similarly to DKIM but has largely been replaced by DKIM. However, Cisco ESA still includes it as an option.