Yes, however there are various ways to go about it. First off, Mac OS X is actually based on a derivative of Unix called Darwin. So merely by opening a Terminal window (type Terminal into Spotlight), you’re using something VERY similar to a Unix bash shell—meaning, you’re using Unix. Linux and Unix are very similar, so by learning one, you are learning many of the essentials about the other. But for various reasons, it’s not always best to work directly on your computer’s native OS when a virtual environment is so easy. So, you can run virtual versions of Linux—not touching your native OS X installation—and its much easier than you think.
If you just want a minimal Linux server (recommended), you can use Levinux, my own Linux distribution. If you want a more complete version of Linux, you can use Parallels, VMWare Fusion or VirtualPC—three different products that allow you to seamlessly run a complete additional operating system—to run more graphical and popular versions of Linux, such as Ubuntu, Fedora, OpenSUSE, Arch or many others. And the final note, is if you’re interested in truly learning Linux, and not just window dressing like Gnome or KDE (making Linux work like a Mac or PC), then what you’re really interested in is Linux Server, and the old-school type-in interface.