Jim - I think the terms get used interchangeably. For instance IBM has a "mini-dock" that is really just a glorified port replicator.... but to me a true docking station is specifically designed for a particular laptop, and utilizes a proprietary system bus and connector and will actually extend the laptop... for instance will have extra drive bays, more USB ports , etc. whereas as port replicator is just a third-party deal that usually plugs in via USB or some other serial port and basically makes it more convenient for you to hook up your cables, but doesn't add much to the mix.
Lots of times cheapie "port replicators" don't support stuff like sound or network, and they're going to be limited by whatever the size of the pipe they're plugged in to
There are some pretty cool docking schemes out there -- the HP widescreen laptops sit in their dock in such a way that you utilize the screen but get the benefit of a real keyboard and mouse, etc. Or the tiny Sharp laptops that sit in their dock almost like a PDA in a cradle.
OTOH some of the true docking stations out there are ridiculously priced - as much as a complete desktop computer. That's why I don't monkey with them. I'd rather spend my $500 on a baseline Dell, and simply remote desktop into my laptop on the network if I want to. Thats' a lot more flexible setup IMO.
((Planning + Process) x Technology) = SUCCESS!