Can someone explain valid shares in a mining pool?
Shares are little packets of work that are sent to every miner on the pool. After your miner completes his piece of work, he send it back to the pool, where it is either accepted or (much less often) rejected. The basic principle is More Hashrate = More Shares, and thus shares are an effective way to determine the work being done by any given miner on the pool.
Valid shares are what a typical miner/mining rig will produce 98% of the time if there are no hardware errors or other technical problems with the miner.
Shares are the means by which most pools determine 'who gets how many coins', since a pool has to split of the proceeds of a block they find.
If two people are mining on a proportional pool, and dude A has 40 Mh/s while dude B has 60 Mh/s,, chances are good that dude A will have submitted 40% of the total shares for that round, and dude B probably had 60% of the total shares. So, when either one of them finds a block worth 100 coins, the pool gives dude A 40 coins and dude B gets 60 coins.
That is the general idea behind the method of shares.