There are two types of participants with different responsibilities on the Ethereum network, validators and full nodes. All validators must run, or be connected to, a full node, but full nodes do not need to be validators. Validators are responsible for aggregating new batches of transactions, or blocks, and proposing them to others within the network, adding to the Ethereum transaction history or blockchain. The network only accepts proposed blocks if ⅔ of the validator sets votes and agrees on the validity of those blocks. These blocks can then easily be verified by full nodes (other network participants).
Today, becoming a validator requires 32 ETH and approximately $400 to $1,000 worth of hardware. Validating on the Ethereum network is a game of luck as validators are randomly chosen with each validator (32 ETH) roughly equivalent to one raffle ticket.
Full nodes complement validators by receiving, validating and relaying proposed blocks and unvalidated transactions. More specifically, full nodes will listen for blocks proposed by validators, verify whether or not these blocks are valid and relay those blocks that meet their requirements. In this way, full nodes are responsible for enforcing the protocol rules and ensuring only valid blocks are shared throughout the network.
In contrast to being a validator, running a full node on the Ethereum network does not require any ETH. It requires only basic machinery and internet connectivity. This encourages users to run full nodes because it limits the costs incurred.