More simplistically it is just a "normal" pool where the miner is smart. Like in p2pool the miner chooses it own transactions. How it differs from p2pool is you still have a traditional pool front end to handle rewards, manage accounts, check stats, etc.
A hybrid pool could integrate into p2pool being a super node to reduce variance (for both the pool & p2pool).
Yes like p2poolparty but one doesn't need to use p2pool to be a hybrid pool. p2poolparty is a good example though. Yeah it is centralized but its power is very limited. It could ripoff its users but it couldn't attack Bitcoin. It couldn't generate double spends, it couldn't participate in a Finney attack.