I've never used kubuntu or ubuntu before but I'll give the rest of the questions a go.
linux is an open source project, its free for anyone to use or change, if they wish to. over time it has become very robust, very versatile and very useful. if learned properly, linux is a very powerful tool. part of the reason for this usability is the fact that it's free for anyone to change and use. http://en.wikipedia.org/wiki/Linux
a distro is a package that works around the kernel to make it more user friendly. all distributions have the same kernel, though version numbers may differ depending on release date. each distro comes with user interfaces, programs and packages that sometimes greatly extend the usability of the os itself. some are better than others in some features and sometimes its just what the user likes to have. different distros can behave in different ways, use different package management systems, etc. but almost all of them these days are designed for ease of use.
the open source nature of linux means that distros are updated frequently. the most recent version of fedora core is fedora core 4, slackware 10.1, etc. it's not really a matter of what's newest, but simply what you feel more comfortable using. ubuntu is a newbie friendly distro, as is fedora core.
to get a feel for linux, there are live-cd's available on the net to download so that you can boot linux over your windows install and try it without committing to the install. a good one is knoppix.