G, VG, F, VF, XF, AU and UNC are grades, signifying how a particular coin is worn and how much of the original details remain. Those letters are abbreviations for:
There are also lesser grades, not represented on Numista:
On Numista coin pages, you can also encounter Proof and BU coins. Both are made a little differently from standard coinage. Details can differ from mint to mint for both.
You can read more about coin grading on Wikipedia: https://en.wikipedia.org/wiki/Coin_grading