O que é uma exchange de Bitcoin?

Could you please elaborate on what a bitcoin exchange is? In simple terms, I'm curious to understand how it functions within the cryptocurrency landscape. Is it a platform where individuals can buy, sell, or trade Bitcoin with other digital currencies or fiat currencies? Does it involve a centralized system, or is it decentralized? What are the key factors one should consider when choosing a Bitcoin exchange? And finally, how secure are these exchanges in terms of protecting user funds and personal information?