Their customisation is what makes them different. For example, Mandrake has drakconf for configuration (something like Control Panel) in Windows. Fedora will have something different like Control Centre. The interface will be different, applications will differ.
Take a look at Linspire. They have tried to make it look and behave like Windows as much as possible
And yes, you can see the source code of any distro. They all are release under GNU/GPL and they have to make their code available otherwise they can't use the Linux kernel and other GNU products.