A live disc is basically the flavour of LINUX you want on a CD/DVD or USB stick. When your computer boots up, choose to run the USB or CD and this will allow you to run your Ubuntu etc straight from the CD rather than having to install it first. Warning though, it can be slow to load up on an aging machine and obviously you won't have all the drivers you need but it will run and a rough idea of what to expect.
I read somewhere, that they're doing anyway with menus in Ubuntu and just using a search bar to navigate your way around the OS and find programs etc. Not sure if I'd like that but until I try it, I suppose it's difficult to make an informed judgement.
It's actually good way to experiment with different versions, that way, you can pick something you like the look and feel of first.
Examples would be: SUSE, Knopix, Ubuntu etc. Just Google Linux distros to find out more.
I actually quite liked Knopix and I had SUSE running on the PS3, until Firmware patch 3.xx disabled the feature, they used to help sell the product.