What is Coinbase and how does it work?

Could you elaborate on Coinbase, its function, and how it operates within the cryptocurrency ecosystem? Specifically, I'm interested in understanding the role it plays in facilitating the buying, selling, and trading of digital currencies. What makes Coinbase unique compared to other cryptocurrency platforms? How does the process of setting up an account and initiating transactions work? And what security measures does Coinbase employ to protect its users' assets? I'm keen to gain a comprehensive understanding of how Coinbase operates and its significance in the cryptocurrency market.