Financial jargon.

Buy-side are traditionally firms that are the "leaf" of the system: investment funds, hedge funds, etc. These are buyers of services.

Sell side refer to firms that are selling a service for the buy side, brokers, banks, etc.

Yes Hedge Funds usually. Sell side is banks selling services and the buyers are Hedge Funds, Asset Managers etc.

Yeah, funds, or anyone who makes investments essentially. Unlike banks, which focus on the sale and generating revenue from fees etc. rather than investment insight. Investopedia isn't always the best source, but for definitions they come in handy: https://www.investopedia.com/terms/b/buyside.asp

(I had a brief internship in finance on the buy-side and had to look up a ton of terms like that, to the degree that I was given this tongue-in-cheek "guide": https://www.amazon.com/Damn-Feels-Good-Be-Banker/dp/14013096.... It's immature af, but managed to teach me the cliches and terminology.)