Rinkeby (https://rinkeby.etherscan.io/), like Kovan, is a Proof-of-Authority (PoA) network, which means that new blocks are not publicly mined as in a PoW network, but are created by trusted nodes in the network at regular time intervals. This makes PoA networks immune to spam attacks, and generally more stable. Rinkeby was started by the Ethereum team, and, as such, is only supported by the Geth client.