What Is the Difference between a “Share” and a “Valid Block Solution”?

A "share" is a partial proof-of-work that satisfies the pool's relatively low target difficulty, proving the miner is actively working. A "valid block solution" is a proof-of-work that satisfies the extremely high difficulty requirement of the entire blockchain network.

A valid block solution is always a valid share, but a share is almost never a valid block solution. Only a valid block solution earns the block reward for the pool.