What Is the Primary Difference between a ‘Mempool’ and a ‘Block’?

The Mempool is a temporary, constantly changing holding area for unconfirmed transactions on a node's memory. It is unique to each node.

A Block, conversely, is a permanent, immutable data structure containing a confirmed set of transactions. Once a block is mined and added to the blockchain, its contents are final and distributed across the network.

The Mempool is the 'waiting room'; the Block is the 'final ledger entry'.