The domain-name option specifies the domain name that client should use when resolving hostnames via the Domain Name System. This is used for example in organizations where there are many workstations and in order to identify them easily their hostnames have appended the domain name like client1.example.org, client2.example.org, server1.example.org and so on.
The domain-name-servers option specifies a list of Domain Name System (STD 13, RFC 1035) name servers available to the client. Servers should be listed in order of preference. This option is pushed by the DHCP server to clients to allow them to query the local DNS servers when they access the Internet.
A DHCP server can push a lot more options than an IP address + subnet mask and gateway. You can check the man page to see all available DHCP options: http://linux.die.net/man/5/dhcp-options