Node operators are the most important aspect of the Chainlink network, they are the bridge between external data and the actual smart contracts on the network.
Nodes don't necessarily need one another. A single node can run by itself or be a part of several different networks. It can restrict individual nodes from operating in their network and choose how data is accumulated and obtained.
Chainlink Software is open source software ran by nodes to bridge off-chain and on-chain environments
Oracle on-chain smart contract detects any data requests and provides a response towards the smart contract requesting said data.
Data Source Subscriptions are the APIs that oracles rely on for data fetched for a smart contract.
External monitoring systems keep an eye on the accuracy and performance of nodes in real-time.