Tasse - 120ml (4oz-ish), this is historically the base UOM of "coffee cup" originally encountered on coffee things. Often carelessly translated to "cup" in countries new to coffee, regardless of overlap with existing alternate meanings of "cup" as a measurement.
Demitasse - 60ml (2oz-ish), a cup for espresso.
Mug - Wikipedia says 8-12oz.
The USA definition of "coffee cup" as an amount one would serve themselves ranges anywhere from 8oz through over 20oz, depending on who you ask. USA-made coffee machine markings also vary, but are always smaller than consumers expect. Currently Black and Decker's "cup" is 5oz and Bunn's is 7oz, which means Bunn's standard 10 cup machine makes substantially more coffee than B&D's standard 12 cup machine.
Unless of course it is a cup of coffee in a recipe, in which case consider the provenance of your recipe and which standardized (or not...) version of a "cup" measurement is indicated before proceeding: https://en.wikipedia.org/wiki/Cup_(unit)
The sizes of "cups" when it comes to coffee seems to be a bit chaotic.