The suffix characters a chip manufacture uses generally are specific to each manufacturer and the datasheet for the chip will give you that information. Check the below datasheet link. Note on the first page the 'ordering information' table. It shows the various temperature ranges and chip packaging options avalible and the suffix characters to select which one to order to meet your requirements.
Specifically for that chip "CNG" means it comes in a 0°C to +70°C operating temperature range and is packaged in a 24 Narrow Plastic DIP package.http://datasheet.octopart.com/MAX7219CNG-Maxim-datasheet-122478.pdf