Blocks hold batches of valid transactions that are hashed and encoded into a Merkle tree. Each new block includes the cryptographic hash of the prior block in the blockchainA blockchain is a shared digital ledger, or a continually up... More, linking the two. The linked blocks form a chain. This iterative process confirms the integrity of the previous block, all the way back to the original genesis block.
On rare occassion, two miners may find the next block at the same (or nearly the same) time (e.g. blocks A and B), creating a temporary fork. Some miners would accept block A that was broadcast to them first, while other miners would accept block B that was broadcasted to them first. Miners accepting block A would try to mine the next block on top of A, while other miners accepting block B would try to mine the next block on top of B. If the next block to be found is on the top of A, then miners mining on the B block will turn to the A chain, which is now the longest chain, the one with the most cumulative proof-of-work and thus considered the valid one by the network. Consequently, the block B is named as an orphaned block and all transactions will return to to the mempool, to be selected and included in the next block(s).