Domain Name Server
DNS is a server hosted normally at your ISP (Internet Service Provider) to let other machines look up ip addresses of your domain.
'Smart host' is a common name in email systems and describes another SMTP server that will be able to relay your messages. I.e: It could accept messages from your internal email server and then deliver it to the final destination. Your ISP will normally provide you with a smtp server you can use as smar thost.