In a lot of the pool threads, I come across the term "Pool Hoppers" in a negative connotation.
What does it mean, and why is it bad?
A pool hopper is someone who times their contributions of shares to a pool such that he obtains an expected higher than average payout per share sent to that pool (sending his share to other pools or solo mining when the expected payout per share is below average). It's bad because it reduces the average payout to everyone else. Note that pool payout schemes can be adjusted to make pool hopping impossible and pay per share pools are immune to pool hopping.
Here's a simple way to understand it. Say a pool pays out 50 BTC divided by the number of shares it receives while searching for a block. Say it will need, on average, X shares to find a block. If it already has 1,000 shares towards finding a block, it will still need, on average, X more to find that block.
So, if a pool has accumulated N shares towards a block, we expect it will need N+X shares to find a block. So the expected payout per share is 50/[N+X]. This means that the longer the pool goes without finding a block, the less each share contributed towards that block is worth. Pool hoppers will abandon a pool as soon as N exceeds about 40% of X.