A block that is compulsory for a miner to have, so that he can mine and earn rewards.
A Candidate block is a requirement for all miners to have, so that rewards are earned as they mine. A candidate block is considered temporary because it can either be removed or validated. Miners have to create a candidate block if they want to participate in this mining competition, although they(miners) constantly compete with one another to be the first whose block is added to the blockchain after validation.
The Merkle tree is formed from the memory pool, after a careful selection of unconfirmed transactions by miners. The block hash is then created by including a nonce and the root hash, after which it passes through several other nodes of the network, to be considered valid and added to the blockchain.