Besu is an open-source Ethereum client developed in Java under the Apache 2.0 license. It supports both public networks, including Ethereum Mainnet and testnets like Sepolia, and private, permissioned networks. The client offers various consensus algorithms such as Proof of Stake, IBFT 2.0, QBFT, and Clique, along with features like JSON-RPC APIs, permissioning, and privacy enhancements.