Domain concept?

[Sdävtaker]

Hello people, i hope you can enlaight me a little.
I installed my DFBSD, configured it, instelled a lot of services 
(apache22, sendmail, imap, dns, pop3, webmail, php4, php5, perl, some 
CMSs, svn, etc...) after that i got a static /30 to make my own DNS real.
In the time i did all this I got in front of 2 concepts "host" and 
"domain" several times. I found a lot of resourses about how go around 
the problems i found, but i could never really understand where is the 
line between them, and what each of them means.
Can u please point me to some place with a nice explanation? I am 
googling it but only find some technical stuff about them and no the 
concept involved.
Thanks for any help
Sdäv