What are the differences?

What are the differences between an 'Inaccuracy' and a 'Mistake', I'm not sure I fully understand.

I'm curious as well.

Inaccuracies are moves that are considered playable and even good just not the most accurate and mistakes or blunders are moves that give up material for no compensation. That is my understanding anyhow but I probably wouldn't keep too much stock in me haha.

lichess probably goes mostly on centipawn loss between your move and the computer's best move. An inaccuracy involves a smaller centipawn loss than a mistake or blunder. Some inaccuracies are bigger deals than others.

For example, an inaccuracy in the opening could be insignificant (if you're following theory), or it could mean you've made what a computer or GM would call a significant error in the opening (but which isn't at all immediately losing), or anything in between. An inaccuracy in a completely winning position is usually insignificant, as your winning plan and the computer's winning plan might both be reasonable and winning. In a tough middlegame position, an inaccuracy is probably most significant. These are the inaccuracies I think we can typically learn the most from.

As far as I know here on lichess it is pure mathematics:

inacc. = 0.5-1 deviation (EngineProposal-Your move)
mistake = 1-2 deviation
blunder > 2 deviation

(you'll find the exact values somewhere in the forum or the FAQ)