Really interested to learn those two terms - "market maker", "liquidity provider". I think I kinda understand the second one (traders who are bringing money from outside the market). Does anyone know where I can read more about this kind of thing?
A market maker is an entity which isn't interested in the underlying assets at all. Their goal is to always end with a 0 asset, all cash balance sheet (over some small time interval). Their goal is to buy X at $100 and sell it at $100.01 a million times a day.
`Liquidity providing` is the side of the trade which had a `resting order` on the order book which traded against a new order (the liquidity taker). Various exchanges have various different fee schedules for your designation as a market maker, how much volume you do, whether you are taking or supplying liquidity, and what kind of order you are placing.
I'm pretty sure Wikipedia has you covered... But TL;DR: traders that always have a buy and sell transaction active, making sure the market allows to buy/sell instantly at any time.