iMail (most commonly know as MacMail) is an email program that works with POP3 and IMAP email accounts.
If you have a gmail account, just type in your email address into the new account dialog and it sets it up for you.
.mac is no longer made, it is now MobileMe.
If you want to make a new MobileMe account, you have to subscribe ($99 a year i think) to mobile me, and get the email account as well as all the other services.
look at me.com