Some countries are part of Schengen and not the European Union. This means a Schengen visa would cover those nations whereas ETIAS would not.
These nations are Iceland, Norway, Switzerland, Liechtenstein
Some countries are part of the European Union are not Schengen. This means an ETIAS visa would cover those nations whereas Schengen would not. These nations are United Kingdom (subject to change due to brexit), Republic of Ireland, Cyprus, Croatia, Bulgaria (in the process of joining Schengen), Romania (in the process of joining Schengen).