Ask HN: What Linux-distro to learn about Linux?

I want to learn about Linux. I am a fairly experienced programmer that can program in Python, Java, C/C++, a little LISP an Ruby and very minor experience in other languages(for example Pascal!).

anyway i want to try out Linux, maybe program a little and surfing but mainly test out the OS.

so what distro is small and easy to play with? and free as in gratis.

and about installing. is there a good guide that takes you through everyhting from partition the harddrive to installing Linux to basic stuff you can do?