Reverse DNS, sending emails and hostname

Question

I don't understand this "reverse dns" thing at all.

So, I have a website - www.someurl.com, and I have an ip address - http://180.160.160.190 (fake). Now, I want to setup a "reverse dns" thing, so that emails that I send out won't be marked as spam.

Questions!

1. why do i have to setup a "reverse dns" thing? What is it, and why do gmail, hotmail et al care about it?
2. all the examples online say I need to setup the "hostname" as mail.someurl.com (well, they say mail.domainname.com, but I cleverly solved for x). What the heck is a "hostname" and what is a "url"? The wikipedia page for hostname left me v. confused.
3. is a url a domain name? it sure isn't a hostname (i hope).
4. back to the mail.someurl.com hostname - why is it "mail" in the examples? Can I fecklessly set the value to "hello" or "mooseymooseymoose" (so it would be mooseymooseymoose.someurl.com)? Does it have any relevance to anything at all?
5. if i am sending emails out as no-reply@someurl.com, does that have any bearing on the first bit of the hostname? that is, should they come from "mail@someurl.com", given a hostname of mail.someurl.com?

cheers, andrew