There are so many flavours of Linux system. Some of them r, Red Hat, SuSE, Debian, Fedora, CentOS etc.etc….. There are also Server and Desktop versions. The variant of Linux has various GUI looks though there core architecture/ basic commandline is same, i.e., Linux Kernel. Such RedHat has one kind of GUI looks while SuSE has another kind of GUI tools / looks. You know UNIX and it gives you extra mileage to learn Linux. Linux is a UNIX like OS. In respect of overall Linux system you can visit here.
I found two more links that will help you to understand linux: