I am trying to figure out which linux I shoud install. Redhat, Mandrake, SuSE, Fedora... There are so many Linuxs, Who can tell me what's the difference between them?

I am a student in CS. I install linux for academic purpose. Thanks

There is actually only one linux. You are looking for a linux distribution:
http://wiki.linuxquestions.org/wiki/.../distribution?

Which one you choose will most likely depend on the purpose of your system - ie a desktop, server or firewall and also what hardware you have.

the best thing i can suggest is read the reviews ...
http://www.linuxquestions.org/reviews/index.php

cause the best way to select a distro is by testing different ones .. me like many others have tried many many distros until we agree upon one and thats what we use ... sometimes use multiple ones ...
the main difference is what apps, etc come packed with the distros ...
cause all i can say is linux is linux ..
linux is the kernel not the distro on a hole...

anyways the most help you will get from this is people telling you what they like
best, so in the end you will still have the same list that you presented us in your
original post ... so go read and decide for yourself is my expert opinion

but just to throw you a recommendation i will say slackware

Slackware will force you to learn all about Linux and if you are a CS major then that is a good thing ! You can buy Slackware on a CD for about 11-12$ or download it from an FTP server.

I think Mandrake or SuSE would be best choices for newbie.