Q: If I wanted to try out various different Linux and bought a second HD for this purpose, could I partition it into 10 or 20 partitions and load 10 or 20 different versions of Linux and be able to boot into any of them as I choose? If so, what would be the easiest way of partitioning it and installing the various Linux ?

A: You can, you'd have to edit a lot of GRUB files as most distros will assume they are the only Linux system on the drive until you manually add the name and location of any other distros. Alternatively you may find that some of them will detect the others happily enough.
You could also use LiveCD's or LiveDVD's, or a virtulisation program such as VirtualBox.

GRUB is a bootloader, it sits at the very beginning of your hard drive, it is the first thing to be loaded when your computer starts and will display a menu (menu contents are held in a file under the OS that installed GRUB), the menu is a list of links which will boot an OS.