What is the difference between ETH and ERC20?

Could you please elaborate on the fundamental differences between ETH and ERC20? I've been hearing a lot about these two terms in the cryptocurrency space, but I'm still trying to grasp the nuances. ETH, I understand, is the native cryptocurrency of the Ethereum blockchain, powering transactions and smart contracts. But what exactly is ERC20? Is it another type of cryptocurrency, or does it serve a different purpose? I'm particularly interested in understanding how these two entities interact with each other and how they fit into the broader cryptocurrency ecosystem. Thank you for your assistance in clarifying these concepts.