Red Hat and Mandrake are distributions of Linux. There are probably over a hundred different distributions of Linux available, although I suspect less than 20 are really geared toward everyday home use (could be wrong there).
To the best of my knowledge there is no 'Linux', there is the Linux kernel and there are distributions based on the Linux kernel.
Different distributions have different types of software/tools/interfaces ect...
Originally posted by Ali Gilani: So why does Google use linux and not RH, or mandrake?
Google use linux...it means their OS is linux based.
- Linux is the kernel (www.kernel.org), it's not an application, it's not a library. You can't run just linux alone (as much as you can't run kernel32.dll alone) - distribution like RH, mandrake, suse (and tons of other at distrowatch.com) all use (approximately) the same linux kernel, but the difference is at the application level, they bundle application in a different way, ...
What you have to understand is that linux is the name of the core operating system.
If you want to modify linux ... Said like that you mean modify the kernel, the core of the OS. For that you need to learn C. If you want to be able to hack applications also, C++ maybe useful for KDE related apps, otherwise C is what you need.