.ETH domains belong to the Ethereum Name System (ENS). The ENS is a distributed domain name system solution for the blockchain, allowing for readable addresses such as david.eth or company.eth instead of using long strings which are confusing.
.ETH domains can be used for an Ethereum-Wallet in order to send or receive money, a smart contract or a swarm hosted site.
ENS is built on smart contracts on the Ethereum blockchain, meaning it doesn't suffer from the insecurity of the DNS system. You can be confident names you enter work the way their owner intended.