Block Data refers to the list of valid transactions included in the block. These transactions are placed in the Merkle Tree to obtain hash of block data, called Merkle Root.
Block Header refers to metadata and is used to identify the block. It contains previous blockhash, timestamp, difficulty, merkle root and nonce.
Block Reward refers to the Cryptocurrency awarded to the mining node (miner) for solving the cryptographic hashing problem to mine a new block. The current Block Reward for Bitcoin is 6.25 BTC and Ethereum is 2 ETH.