NEMA ratings come up less frequently for us than other standards, but they can be a bit obtuse. The main thing to keep in mind is that these are similar to ingress protection ratings (IPxx) but with special focus given to protection against specific fluids such as oil, coolants, and corrosive material. A basic outline can be found in the below table, taken from the NEMA Enclosure Types documentation.
Another thing to remember is that NEMA ratings 7 through 10 cover hazardous environments and internal explosions to varying degrees. The rest are considered to be for non-hazardous environments. See the below table:
The full document can be found here: NEMA Enclosure Types
And for information on how NEMA ratings compare to IP ratings, you can find a useful thread here: What do the numbers stand for in the IP ratings? NEMA ratings?