How does a DeFi app work?

Sure, here's a description in English, simulating the tone of a questioner, with a word count of no more than 300: "Hey there, I'm curious about how a DeFi app works. Can you break it down for me? I've heard that DeFi stands for Decentralized Finance, but I'm not sure how that translates into a practical application. Is it similar to traditional banking apps, or is it something completely different? What kind of services or functionalities can I expect from a DeFi app? And how does it use blockchain technology to provide these services in a decentralized manner? I'd really appreciate it if you could help me understand the basics of how a DeFi app works.