I'll give an example of how the website has been useful in my own research. I'm currently researching boars in Greco-Roman iconography, and wanted to identify Greco-Roman coins with a boar on them. My friend Ted Erho told me about the CNG website, so I went to check it out. I used their research page which allows me to search historical auctions. I typed "boar" in the search field and got 1,543 hits, 6 of which were historical articles. Not all of these hits were useful for my purposes, but a great deal of them were. I was able to find some very interesting and helpful specimens, like this beautiful coin from Apulia with a boar on the reverse:
The entry for the coin gave me a basic description that included the catalogue number:
APULIA, Arpi. Circa 325-275 BC. Æ (22mm, 7.53 g, 6h). Laureate head of Zeus left; thunderbolt to right / Boar standing right; above, spearhead right. HN Italy 642; SNG ANS 635. VF, even brown patina.
I'm newer to using coins in my research, so I don't necessarily know where to find "HN Italy 642" or "SNG ANS 635." But it was easy enough to find out. By clicking the "Bibliography" tab I was able to search for "HN Italy" and determine that it refers to:
N.K. Rutter, ed. Historia Numorum. Italy. London. 2001
Now I know which catalogue to find it in at my library! Having the catalogue number also allowed me to do another CNG search to find other specimens of this coin. Typing HN Italy 642 into the search field brought up a few more examples, like this one:
The coin is from Apulia, a region in Italy, and it has Zeus, perhaps the Calydonian boar, and Greek letters. Italian coins with Greek figures, themes, and letters is pretty interesting to me, so I did an additional CNG search for "Apulia" just for fun, and came up with some coins like this:
CNG is one of the many useful Internet resources for ancient coins, try taking it for a spin!