Linux is just a kernel. It's the packages developed in the Open source world that build around the kernel, that make it into a fully fledged OS. And since it's Open Source, everyone has thier own take on how to do things, so each distro, while using the same kernel (essentially) will be setup differently than the next.
Anyway, linux is linux, most distros will be mostly compatable with eachother.
Checkout the various live CDs out there to get a taste for it. Knoppix, Ubuntu, DSL (Damn Small Linux), etc...