Hostname : A hostname is a one through which a computer is being identified. U can say that its the name of ur computer.
DHCP : DHCP is known as Dynamic Host Configuration Protocol. This is actually used to assign IP adrresses to the computers
dynamically. U can find the whole documentation on redhat site.
Domain Name : U will find the detail information here http://www.webfoot.com/advice/email.domain.html?Email
If any queries feel free to ask.