Stealing this fine answer from Google:
Ethereum is a platform for deploying smart contracts, or pieces of logic that control the movement of native assets or state on the single Ethereum chain. In contrast, Polkadot aims to provide a framework for building your own blockchain and an ability to connect different blockchains with each other.
As for the DeFi part: I actually don’t know the difference. But I guess yeah, hopefully the fees will be lower🙏🏼🙏🏼🙏🏼😆