Action game vs RPG?
It is a sliding scale, but of what?
Simple, actually. In an action game, the performance of the game avatar is directly related to the physical skill of the operator. If you suck then your character suck, if you are godly then you play godly.
Example? CoD.
In an RPG, the performance of the game avatar is related to the limits of the game character's own stats. Accuracy is based on stat, luck is based on stats, charisma is based on stats, damage is based on stats. You can make a difference by being skillful, but it is much less of a factor than growing the character themselves.
A classic example is TES Daggerfall, where you can swing your sword as fast as you like but the chance to hit is based on the stats and not how you aim.
A lost of games these days have a mix of the two extremes, but often the RPG element is just window dressing. This is most obvious where games have level scaling, where the game enemies get stronger the stronger the player character is. This is when you have an Action game that wants to PRETEND it is an RPG, so it has a veneer of RPG skin that end up being just lies and deceits. The level ups are fake, just put there for appearances.
ironically a really old example of fake level-ups is in the arcades D&D licenced beat-them-ups, where you see cut-scenes between levels that tell you the characters have gotten stronger, when in reality they have not. The enemies are the same difficulty all the way through, they just SAY the characters are stronger. That is the funny reverse case where the game is trying to be an RPG when they were an action game due to hardware restrictions.
So yeah, most games these days are just actions games, because most gamers want their skills to matter. Older gamers like me who aren't as young as they used to would prefer RPGs that don't have real life reflexes or memorize button presses. But as we get older such games are harder and harder to find.