How do you get a shell on a Mac?

You're correct, it is a descendant of Unix. Mac calls the shell "Terminal" and you can find it under Applications → Utilities → Terminal:

Another way is to use Spotlight to launch Terminal.

1. Press command-space to open the Spotlight search box in the upper right-hand corner
2. Begin typing "Terminal"
3. Click on Terminal or just hit return if it's the first result.

I suggest downloading iTerm. It has a few more extra features than Terminal included with Mac OS X. Among them are tabs, full screen mode, transparency and background images, VT100 emulation with colours, and more.

As was mentioned, previously you could use the Terminal (see instructions above) or you could also use xterm. They both have their benefits and their flaws ... so you can give them a whirl and see which you prefer.

I ended up using xterm more whenever I needed to code because I couldn't get the built-in Mac terminal to work with mouse clicks and other shortcuts which I use while coding in vim.