In general, a "dual boot" refers to having multiple operating systems on the computer....you start your computer, then select which one to run...for Macs I think you have to use Bootcamp in order to do this....

Virtualization is when one operating is ran "inside" another operating system...it generally is not as stable as a dual boot and is more resource intensive...however, a dual boot requires your hardrive to be partioned such that each operating system has its own "section" of the hardrive...if your hardrive isn't already partitioned, you will probably have to reinstall your mac OS in order to partition it...

So, if your hardrive is already partioned, you have an external hardrive, or you don't mind reinstalling your operating system, then dual boot is the way to go...

If you have a pretty good computer (mainly processor and RAM) and can't (or don't feel like) do the above, a virtual machine (like Parallels) is your best choice...