Google Chrome is an internet browser (or navigator) based on the free software project Chromium. Chrome is long available on Windows, but only really recently is it available on Mac OS X and Linux, though still under “beta” (meaning testing/experimental) stage.
I have been using Chrome or Chromium without any obvious differences (at least on Linux).
So why Chrome/Chromium when there is already Firefox? First it is a matter of choice, I could answer you why so many different vehicles? Second, I like the look and feel of Chrome, it optimises the use of screen real estate, it is not cluttered with many menus or actions on the toolbar and it seems to always know where I want a new tab to be opened. Third, it launches really fast.