All miners on the pool are working on the transactions that the pool has selected for inclusion in the next block.  Different pools will normally have a slight difference in the transactions in their blocks due to different size limits, and which transactions they have seen on the network (since it's all p2p, not all pools since all transactions at the same time).

BTC Guild sends you a new work template every 30 seconds, regardless of whether or not a new block is on the network.  This new template includes an updated list of transactions to include in the block, since the more time between blocks, the more likely new transactions with higher priority/higher fees have been sent on the network waiting for confirmation.