BTC Relay is a building block that allows Ethereum contracts to securely verify Bitcoin transactions without any intermediaries: in short users can pay with Bitcoin to use Ethereum DAPPs.
BTC Relay is an Ethereum contract that stores Bitcoin block headers. BTC Relay uses these block headers to build a mini-version of the Bitcoin blockchain: a method used by Bitcoin SPV light wallets.
BTC Relay is open source, trust-free, and decentralized.