Actually, viton is a material out of which they are made.
It's big advantage is that it's impervious to CO2 ( which buna-N or nitril is not ) but it's much, much more expensive by comparison, and viton does NOT like water !
The size is the "dash number" because the actual technical designations include a number of digits prior to the dash that are ( mostly ) irrelevant to us.
Durometer is a measure of hardness. 70 is a typical number for fawcet washers and such, though they can be had ( for a price ) at nearly any durometer one would like.
Lower durometer deform ( and therefore seal ) better at low pressures, while higher durometers deform at higher pressures. Consequently, low durometer doesn't work well at high pressures, because it extrudes out of the sealing space, and high durometer doesn't work at low pressure, because it doesn't deform at all, and therefore doesn't seal at all.
Buna-N ( aka nitril ) is the most common material out there, and cheapest.