Both Mac OS X and Ubuntu (and other Linus OS) has terminal / command line.
Are these terminal identical across different OS? If not, what are the important difference?
Terminals used to be real machines, e.g. a monitor with a keyboard attached, which sent your keystrokes to the actual computer at the other end, and receiving output from this computer. For example, the famous VT100:
Nowadays, this is emulated by those applications, like the Terminal.app on OS X, or GNOME Terminal for the GNOME graphical interface on Linux distributions.
As it's decoupled from the shell that you can run (e.g. bash, zsh, ...), you are free to use any other terminal emulator. For example, on OS X, you can use iTerm 2, which offers a bit more than the normal terminal application.
As the terminal is the bridge between your GUI and the text-only shell, its functionality should go beyond accepting keyboard input and displaying output from the shell. A terminal should supply you with means of:
To sum it up: Different terminals offer different functionality with regards to how they interact with your operating system on the one hand, and the shell on the other hand. You can find a comparison of terminal emulators on Wikipedia.
That being said, there isn't an important difference. Most people set up terminals to their liking, e.g. some like a transparent background, whereas others don't. Some like them to display their shell black on white, others rather like it green on black.**
* Note that this is a rather simplified statement. The terminal just needs to find a way to map special keys – especially on Macs, where there's a
** This is also a functionality of the terminal: Displaying colors. You can actually tweak your terminal to show stuff the shell intended to be green as red instead. It's primarily a matter of customization.
note: the terminal is a wrapper program which runs the shell. the term derives from times where it still was a physical device (keyboard).
the shell (or command interpreter) runs commands and returns the results. you basically can run the same shells on both operating systems:
shells of the mentioned operating systems (in question tags) are all using the same core commands because they are UNIX-like and have to fulfill the POSIX standard. this is why you will feel familiar on both OSs.