What is the difference between apple and Apple? See also my past answer:
what is the difference between the class and encapsulation in programming[
^].
Isn't this quite obvious: such questions are not valid. This question in not completely incorrect, but can be greatly improved. You should have re-formulated it:
"What are Wikipedia, Google, CodeProject and StackOverflow.com?"If you asked and got good Answers, the Questions like your original one would not be needed!
[EDIT]
On second thought, as I can just found that you are not along, I'll help to find the answers (not "difference", of course), to start with.
"Host" is a computer:
http://en.wikipedia.org/wiki/Host_%28network%29[
^].
And "domain" in computing has several different, often unrelated meanings:
http://en.wikipedia.org/wiki/Domain[
^].
—SA