Shares are the work being sent to the miner and results sent from the miner back to the pool. They are one-shot data transfers, with the pool/miner talking to each other 18x per minute (on average). Read the Workers part of that link for details.
Ok ,I get it ( may be ) :
In theory , every miner must send ALL hashes to the pool , but this needs a very huge bandwidth. So to limit this amount of hashes, the pool applies a FILTER to eliminate non-interressing hashes. The pool sets an initial difficulty to the miner and forces it to send only hashes that meet this difficulty.
The pool adjusts this difficulty according to the performances of the miner ( hashrate ) in order to get 18 shares per minute. In the meantime, this can be a metric to estimate the hash rate of the miner .
Is it like this ?