Are a blunder and mistake different things or just two words for what is the same thing?

That's a classic blunder. Steak should bleed.

Inaccuracies are moves that don't really do much at all, mistakes makes your position worse, blunders makes you lose materials or the game.

OK, thanks for the responses. They variously add up to a good clarification for me at least. Inacccuracies, mistakes, and blunders are essentially the same thing, that is, moves which add up to weakening your chances in the game that

differ in their degree of effect,

Clearly it's a numerical comparison to their engine's determination of one's best move. I assume it marks a move as brilliant when it's better than the move the computer identified as best. (Not sure how their own engine could determine that ... deeper post hoc analysis??)

I'm with you guys, it would be nice to know what those numbers are: inaccuracy = best - 0.5

mistake = best - 1

blunder = best - 2

missed win = best - 3??

Since the people who know aren't weighing in here, I'll try to eyeball the pattern and post again later. 

Mistake (?) changes either a drawn position to a lost position,
or a won position back to a drawn position.
Blunder (??) changes a won position to a lost position.