When using Linux, it is helpful to know the keyboard commands because unlike Windows, GNU/Linux is a command line based OS, and all the fancy GUIs are just that, GUIs. They are not necessary, basically. If you don't know the commands though I found a handy wallpaper that shows several important commands for the terminal and also for VIM (A command line based text editor). I believe these commands also work for the most part on Mac OSX, since that is a UNIX based OS. (I don't expect then all to work though for that.)
You can download the image here.