which linux distributions are similar to one another?
 

centos & fedora are similar? what other distributions are similar?

Go to www.distrowatch.com and see what the base distro is. Anything based on Debian is similar to Debian, same with Red Hat, Slackware, Gentoo and the rest. They will have differences but, at heart, will be similar. Also look at the distro map in my signature, that will tell you visually which distros are rooted in which.

The answer you received above is completely correct on one interpretation of your question (the most obvious interpretation).

If, however, you mean, which distros look most like one another, you will probably find that all distros using a particular GUI (kde 3.5, kde 4, Gnome, xfce, enlightenment, windowmaker...) look more like one another, plus or minus a bit of configuration and a change of wallpaper, than distros using different UIs. Or, at least, with a relatively trivial amount of effort, can be made to look like one another.

I checked about 40 LiveCD-Distros but I found them all very different from each other. If I take especially the ten best once everyone was in someway very special and not to compare with the others.
SliTaz extremely Small
Puppy does everything by beeing small and it is suitable for beginners.
GoboLinux has a completely different file organization
openmamba did a very good job also with the hardware.
and so on and so on.


Ofcorse that KDE is in someway similar to KDE but there are so many more things to consider that I really can't answer that question

what about similarity's in commands? isnt centos and feora have the same commands?

Most basic commands are the same across ALL Linux distros.

You may be interested in this timeline:
http://futurist.se/gldt/