Ask a stupid question, but what are they and what's the difference?
If, for example say, I have a domain www.abc.co.uk which I foolishly had hosted many moons ago by one company, and www.abc.org.uk which I wisely hosted with bodhost.com, could I use one of these options to point any traffic (web, mail, ftp, etc) from .co.uk to org.uk?
For mail you could just setup email forwarders. But you could change the name servers on the domain to bodhost.com then you put it as an addon domain. Or you could just setup a page on the .co.uk site to say to go to such and such a page for new site.