It is indeed as easy as SD says. I have several domains registered which are purely "forwarders" (very cheap, that way!). They forward to different pages on my main ISP. Very easy to do, hard to describe.
It's possible to set them up to show the domain name you want, rather than the one actually connected, in the URL box. Try
EssexMan which is an example of that.
Anyone who really wanted to know could find out quickly where that actually goes, but the casual customer is happy.