These applications create virtual Windows PCs, which allow you to run Windows (and Windows applications) at the same time as OS X and therefore you can switch between them withoout needing to reboot. There is a slight performance hit when running Windows in this way but for most people this is negligible. Also you will need a lot of RAM to run these, at leat 1GB and preferably 2GB of RAM.
Coherence mode on Parallels allows you to run Windows applications seamlessly alongside Mac applications.
Both require a Windows licence, but doesn’t need to be SP2, with Parallels you can even install and run Windows 3.1! Parallels XP support is excellent, however support for Vista is less good, so you can’t use things like Aero for example.
I have installed Leopard on my iMac (using an external Firewire drive rather than my main drive).
It installed fine, though I find it amusing that Apple’s “about one minute” can be longer than any other minute, maybe a Time Machine thing?
I have had a quick look and I am quite impressed. I am quite pleased with the performance on my iMac which seems much faster, certainly don’t get the lag I have been getting when changing the volume for example.
Safari 3 is much better and I might just install 10.4.11 on my other Macs to get Safari 3, like the fact I can move tabs.
File sharing looks relatively simple, but quite different to Tiger therefore new guides are essential.