I want to know Linux in details, but I am confused. I should test everything with
Which one is most widely used in servers so that I can test there?
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Ubuntu is common enough that knowing about it is a good thing in general.
CentOS is, as Chris S said, very common in servers and hosting sites.
RHEL is what people pick when they want a commercially supported Linux, and what most people who sell Linux software certify their stuff against. If you're doing large scale Linux, knowing this is a very good idea.
SUSE.... openSUSE is the desktop distro and is pretty good for figuring out how Suse Linux Enterprise Server works, and is also a KDE distro which is uncommon.
SLES is what Microsoft shops who need Linux use, since Microsoft resells support certificates (the last time I looked at Novell financials, over 50% of their Linux sales came by way of Microsoft). A smaller community of people who want a commercially supported Linux but don't want to deal with RedHat pick SLES. Even so, a lot of Linux software vendors do certify for SLES.
If you just want to know how Linux works at the guts, I recommend Slackware. No one uses it much, but if you get it hammered out you'll know a lot of the fundamentals of how it works. You'll know where all the config files are and what to do with them, how to install software without a package manager (Slackware has one, but the selection is wanting), and a full appreciation of just what high quality dependency-resolution means for RPM or DEB.
Ubuntu is very common in servers, workstations, and desktops. RHEL is very popular in mission critical applications where the company might not necessarily have in-house Gurus. CentOS is very popular in servers, especially Internet services. I don't know anyone who uses Suse these days, but I'm sure there some Netware hold-out lurking.
If you know people who use something, pick-up a copy of what they've got; then you can bounce questions off them.
Like everything else, it depends.
Server-wise I think you'll encounter RedHat (or free variants such as CentOS or Whitebox). In the compute-farm, cad-tool and simulator market you'll encounter these almost exclusively because many large vendors refuse to support their tools running on anything else. This is where I have most of my experience; I think I easilly support twice as many CentOS/RHEL systems as every other flavor put together.
In environments which are less tool-constrained, or where "freedom" is a religion, you'll encounter more debian and debian-type systems. I've somehow managed to avoid any prolonged exposure to debian myself, but there is a fair bit of it out there.
Beyond that, I've occasionally found places which run SuSE/SLES for legacy reasons, and then sliding down the scale you periodically find gentoo shops where everything is rabidly customized.
I'm yet to encounter a site which uses Ubuntu-family systems for servers -- although I frequently find engineers running it at places where they pick, build, and maintain their own workstations.
I think Chris S has the best idea -- use whatever your local guru uses so you can ask him questions. Then once you get a feel for what is going on, start looking at other things.
This is highly subjective but:
If you want to learn Ubuntu, use Ubuntu. If you want to learn RedHat, use RedHat. If you want to learn SuSE, use SuSE.
However, if you really want to learn the complete details of Linux, your best bet would be to go with something really vanilla, like Slackware/Debian. Once you master something really vanilla, you will be able to use any of the distros that you have mentioned, easily.
If you want to learn Linux (at it's pure flavor :) i think the best is LFS linuxfromscratch.org
But like others said, every other distro has its "tweaks" debian distros behave in a way, redhat distros behave in another way, but they are really good, but "not pure".
Some follow SysinitV startup others don't... Some are more "a'like" unix other's don't...
In my opinion if you want to learn and have time try LFS