I've recently bought a used iMac from ebay in order to learn more about these computers. I've learned a few things since buying it. I'm hoping to post on here from time to time various tidbits that I've learned. The ultimate goal is to become certified as Apple Certified Support Professional.
The windowing concept in Mac OS is a little different from Windows, or even X Windows. If you hit the red button at the top left of a window, it doesn't close the application. There's a doc at the bottom of the screen with several common applications, and any running applications. If you look under/in front of an application and see a blue light, that's your indication that the program is still running. Even if there are no open windows, a program will continue to run until you exit it. Two ways that I know about include going to the menu at the top when the program is active, picking the main menu item (Usually the program's name) and then picking the Quit option at the bottom. The other way is to make the program active, then push the Command-Q keyboard shortcut.
On a keyboard with the Windows keys, the Start button will stand in for the Command button on an Apple keyboard.
Here is a list of other keyboard shortcuts.
I like the way Mac OS software is distributed. Mostly in the form of DMG files, which are like disk images. Opening one causes it to show up like a hard drive, or CD. According to information I've found online, they can be compressed or password protected as well. Installing a program can be as simple as downloading a dmg file, mounting it, and then copying the program to the application folder. The dmg file could also have an installer as well.
It was incredibly easy to setup my printer. I've read the backend of the printing support is based on CUPS which also can handle printing in Linux. Common Unix Printing System was originally what it stood for. Mac OS X has a BSD-based kernel named Darwin. That part of the system is released as open source software. It's missing elements required for the GUI part of Mac OS and can't run Mac OS applications when installed separately from Mac OS X. You can interact with the BSD side with the Terminal application in Applications|Utilities.
The default shell for the Terminal is TCSH, but others are supported and included depending on the version of Mac OS X. Bash, csh, ksh, zsh are some of the options available. Looks like according to the Wikipedia article on TCSH, it's no longer the default as of OS X 10.3.. the default shell is BASH. Running "echo $SHELL" on mine seems to back that up.. the result was "/bin/bash"
X11 was already installed on the one I bought, apparently it is no longer included by default in new installs of Mac OS X. I've used this before to open an X session to one of my Ubuntu computers. It's handy to do so in order to run programs on that computer, or manage files on it.
I've played around with various applications and settings so far. I tried out the Speech recognition. It's not very accurate for me, but it is a neat idea. It makes me wonder if later versions of OS X have that supplemented with SIRI or even still have it at all.