ftp addres and username

I have recently and are currently using ftp client to get files from my boss. That's easy. I've got that mastered. Nothing to it.
Now I am doing web pages and am all confused.
The ftp address and username are different than hostgator is telling me they should be.
the ones for my boss are address: www.domainname username is: username@domainname
Host gator is telling me that it should be ftp.domainname and username is simply username, no @domainname.
Which one is correct? Can someone please explain this to me.