Apple Boot Camp
How it works exactly, not so sure. I'm not a Mac user. Perhaps the
Mac users here could you on that.
As for Parallel OS, better known as Parallels Desktop, it's a type
of virtualization, something like VMWare.
Instead of creating many partitions to load an OS, Parallels allow
you to run two or more OS together, one of which is the main, the
others are on a virtual partition.
Their product is here,
and some usage is written at ZDNet.